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

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

this change was created in the same spirit of b8c30a79.

in BlueFS.test_shared_alloc and BlueFS.test_shared_alloc_sparse, we
keep the return value of `fs.get_perf_counters()`, and dereference 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:

```
==548153==ERROR: AddressSanitizer: heap-use-after-free on address 0x6110000336c0 at pc 0x7fc810326654 bp 0x7ffd869be8f0 sp 0x7ffd869be8e8
READ of size 8 at 0x6110000336c0 thread T0
    #0 0x7fc810326653 in ceph::common::PerfCounters::get(int) const /home/jenkins-build/build/workspace/ceph-pull-requests/src/common/perf_counters.cc:246:8
    #1 0x564e7a5397a5 in BlueFS_test_shared_alloc_sparse_Test::TestBody() /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/objectstore/test_bluefs.cc:1265:3
    #2 0x564e7a644006 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 0x564e7a5fdbc2 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 0x564e7a5ae7ec in testing::Test::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2680:5
    #5 0x564e7a5b0822 in testing::TestInfo::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2858:11
    #6 0x564e7a5b1e5b in testing::TestSuite::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:3012:28
    #7 0x564e7a5cf2e8 in testing::internal::UnitTestImpl::RunAllTests() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:5723:44
    #8 0x564e7a64c8b6 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 0x564e7a604662 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 0x564e7a5ce672 in testing::UnitTest::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:5306:10
    #11 0x564e7a55a410 in RUN_ALL_TESTS() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/include/gtest/gtest.h:2486:46
    #12 0x564e7a551295 in main /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/objectstore/test_bluefs.cc:1609:10
    #13 0x7fc80d775d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #14 0x7fc80d775e3f in __libc_start_main csu/../csu/libc-start.c:392:3
    #15 0x564e7a4296a4 in _start (/home/jenkins-build/build/workspace/ceph-pull-requests/build/bin/unittest_bluefs+0x2856a4) (BuildId: fd4e4e0b1c2f9a3b0c1a7051d8ed68b3576e3277)

0x6110000336c0 is located 0 bytes inside of 208-byte region [0x6110000336c0,0x611000033790)
freed by thread T0 here:
    #0 0x564e7a4e7b1d in operator delete(void*) (/home/jenkins-build/build/workspace/ceph-pull-requests/build/bin/unittest_bluefs+0x343b1d) (BuildId: fd4e4e0b1c2f9a3b0c1a7051d8ed68b3576e3277)
    #1 0x564e7a686ce3 in BlueFS::_shutdown_logger() /home/jenkins-build/build/workspace/ceph-pull-requests/src/os/bluestore/BlueFS.cc:462:3
    #2 0x564e7a6a9b55 in BlueFS::umount(bool) /home/jenkins-build/build/workspace/ceph-pull-requests/src/os/bluestore/BlueFS.cc:1076:3
    #3 0x564e7a539767 in BlueFS_test_shared_alloc_sparse_Test::TestBody() /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/objectstore/test_bluefs.cc:1262:6
    #4 0x564e7a644006 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 0x564e7a5fdbc2 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 0x564e7a5ae7ec in testing::Test::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2680:5
    #7 0x564e7a5b0822 in testing::TestInfo::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2858:11
    #8 0x564e7a5b1e5b in testing::TestSuite::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:3012:28
    #9 0x564e7a5cf2e8 in testing::internal::UnitTestImpl::RunAllTests() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:5723:44
    #10 0x564e7a64c8b6 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 0x564e7a604662 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 0x564e7a5ce672 in testing::UnitTest::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:5306:10
    #13 0x564e7a55a410 in RUN_ALL_TESTS() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/include/gtest/gtest.h:2486:46
    #14 0x564e7a551295 in main /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/objectstore/test_bluefs.cc:1609:10
    #15 0x7fc80d775d8f 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>