test/objectstore/test_bluefs: fix heap-use-after-free

test/objectstore/test_bluefs: fix heap-use-after-free

in BlueFS.test_shared_alloc, we keep the return value of
`fs.get_perf_counters()`, and deference it after umounting the fs,
but the `PerfCounters*` pointer returned from `fs.get_perf_counters()`
is destroyed in `BlueFS::_shutdown_logger()` which is in turn called
by `BlueFS::umount()`. so ASan points this out:
```
==1662613==ERROR: AddressSanitizer: heap-use-after-free on address 0x6110000b2d80 at pc 0x7f0eefc30644 bp 0x7ffcdbab6430 sp 0x7ffcdbab6428
READ of size 8 at 0x6110000b2d80 thread T0
    #0 0x7f0eefc30643 in ceph::common::PerfCounters::get(int) const /home/jenkins-build/build/workspace/ceph-pull-requests/src/common/perf_counters.cc:246:8
    #1 0x557595ddfc15 in BlueFS_test_shared_alloc_Test::TestBody() /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/objectstore/test_bluefs.cc:1182:3
    #2 0x557595eeef66 in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2605:10
    #3 0x557595ea8b22 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2641:14
    #4 0x557595e5974c in testing::Test::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2680:5
    #5 0x557595e5b782 in testing::TestInfo::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2858:11
    #6 0x557595e5cdbb in testing::TestSuite::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:3012:28
    #7 0x557595e7a248 in testing::internal::UnitTestImpl::RunAllTests() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:5723:44
    #8 0x557595ef7816 in bool testing::internal::HandleSehExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2605:10
    #9 0x557595eaf5c2 in bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2641:14
    #10 0x557595e795d2 in testing::UnitTest::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:5306:10
    #11 0x557595e05370 in RUN_ALL_TESTS() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/include/gtest/gtest.h:2486:46
    #12 0x557595dfc1f5 in main /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/objectstore/test_bluefs.cc:1603:10
    #13 0x7f0eed083d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #14 0x7f0eed083e3f in __libc_start_main csu/../csu/libc-start.c:392:3
    #15 0x557595cd46a4 in _start (/home/jenkins-build/build/workspace/ceph-pull-requests/build/bin/unittest_bluefs+0x2856a4) (BuildId: 5439261504ca3d7549fe9bcda1d17ef6d4d9b644)

0x6110000b2d80 is located 0 bytes inside of 208-byte region [0x6110000b2d80,0x6110000b2e50)
freed by thread T0 here:
    #0 0x557595d92b1d in operator delete(void*) (/home/jenkins-build/build/workspace/ceph-pull-requests/build/bin/unittest_bluefs+0x343b1d) (BuildId: 5439261504ca3d7549fe9bcda1d17ef6d4d9b644)
    #1 0x557595f31c43 in BlueFS::_shutdown_logger() /home/jenkins-build/build/workspace/ceph-pull-requests/src/os/bluestore/BlueFS.cc:462:3
    #2 0x557595f54ab5 in BlueFS::umount(bool) /home/jenkins-build/build/workspace/ceph-pull-requests/src/os/bluestore/BlueFS.cc:1076:3
    #3 0x557595ddfbd7 in BlueFS_test_shared_alloc_Test::TestBody() /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/objectstore/test_bluefs.cc:1180:6
    #4 0x557595eeef66 in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2605:10
    #5 0x557595ea8b22 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2641:14
    #6 0x557595e5974c in testing::Test::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2680:5
    #7 0x557595e5b782 in testing::TestInfo::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2858:11
    #8 0x557595e5cdbb in testing::TestSuite::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:3012:28
    #9 0x557595e7a248 in testing::internal::UnitTestImpl::RunAllTests() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:5723:44
    #10 0x557595ef7816 in bool testing::internal::HandleSehExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2605:10
    #11 0x557595eaf5c2 in bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2641:14
    #12 0x557595e795d2 in testing::UnitTest::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:5306:10
    #13 0x557595e05370 in RUN_ALL_TESTS() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/include/gtest/gtest.h:2486:46
    #14 0x557595dfc1f5 in main /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/objectstore/test_bluefs.cc:1603:10
    #15 0x7f0eed083d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16

previously allocated by thread T0 here:
    #0 0x557595d922bd in operator new(unsigned long) (/home/jenkins-build/build/workspace/ceph-pull-requests/build/bin/unittest_bluefs+0x3432bd) (BuildId: 5439261504ca3d7549fe9bcda1d17ef6d4d9b644)
    #1 0x7f0eefc33180 in ceph::common::PerfCountersBuilder::PerfCountersBuilder(ceph::common::CephContext*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, int, int) /home/jenkins-build/build/workspace/ceph-pull-requests/src/common/perf_counters.cc:537:21
    #2 0x557595f30ac9 in BlueFS::_init_logger() /home/jenkins-build/build/workspace/ceph-pull-requests/src/os/bluestore/BlueFS.cc:221:23
    #3 0x557595f42bc6 in BlueFS::mount() /home/jenkins-build/build/workspace/ceph-pull-requests/src/os/bluestore/BlueFS.cc:977:3
    #4 0x557595ddd339 in BlueFS_test_shared_alloc_Test::TestBody() /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/objectstore/test_bluefs.cc:1139:3
    #5 0x557595eeef66 in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2605:10
    #6 0x557595ea8b22 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2641:14
    #7 0x557595e5974c in testing::Test::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2680:5
    #8 0x557595e5b782 in testing::TestInfo::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2858:11
    #9 0x557595e5cdbb in testing::TestSuite::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:3012:28
    #10 0x557595e7a248 in testing::internal::UnitTestImpl::RunAllTests() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:5723:44
    #11 0x557595ef7816 in bool testing::internal::HandleSehExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2605:10
    #12 0x557595eaf5c2 in bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2641:14
    #13 0x557595e795d2 in testing::UnitTest::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:5306:10
    #14 0x557595e05370 in RUN_ALL_TESTS() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/include/gtest/gtest.h:2486:46
    #15 0x557595dfc1f5 in main /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/objectstore/test_bluefs.cc:1603:10
    #16 0x7f0eed083d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
```

in this change, instead of keeping `logger` across the `umount()` and
`mount()` calls, we get another instance of `logger`, query it for
the perf counter that we are interested, and compare the value
to see if it is unchanged.

this should address the ASan warning above.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>