crimson/os: fix memory corruption in AlienStore::get_attrs().

crimson/os: fix memory corruption in AlienStore::get_attrs().

`FuturizedStore` and `ObjectStore` use different memory layout for
conveying object attributes: map of `bufferlists` and map of `bptrs`
respectively. Unfortunately, `AlienStore` was trying to solve this
mismatch with just a `reinterpret_cast`.

Very likely this problem was the root cause behind the observed
crashes in `PGBackend::load_matadata` like the following one:

```
2021-06-15T09:25:07.511 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]: DEBUG 2021-06-15 09:24:19,199 [shard 0] osd - peering_event(id=412, detail=PeeringEvent(from=7 pgid=5.14 sent=49 requested=49 evt=epoch_sent: 49 epoch_requested: 49 MInfoRec from 7 info: 5.14( v 45'2 (0'0,45'2] local-lis/les=48/49 n=0 ec=44/44 lis/c=48/44 les/c/f=49/45/0 sis=48) pg_lease_ack(ruub 19.176788330s))): complete
2021-06-15T09:25:07.511 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]: Segmentation fault on shard 0.
2021-06-15T09:25:07.511 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]: Backtrace:
2021-06-15T09:25:07.511 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]:  0# 0x000055C99757FFBF in /usr/bin/ceph-osd
2021-06-15T09:25:07.511 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]:  1# FatalSignal::signaled(int, siginfo_t const*) in /usr/bin/ceph-osd
2021-06-15T09:25:07.511 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]:  2# FatalSignal::install_oneshot_signal_handler<11>()::{lambda(int, siginfo_t*, void*)#1}::_FUN(int, siginfo_t*, void*) in /usr/bin/ceph-osd
2021-06-15T09:25:07.512 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]:  3# 0x00007F34BB632B20 in /lib64/libpthread.so.0
2021-06-15T09:25:07.512 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]:  4# 0x000055C99263D4D2 in /usr/bin/ceph-osd
2021-06-15T09:25:07.512 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]:  5# 0x000055C992740E47 in /usr/bin/ceph-osd
2021-06-15T09:25:07.512 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]:  6# seastar::continuation<seastar::internal::promise_base_with_type<std::unique_ptr<PGBackend::loaded_object_md_t, std::default_delete<PGBackend::loaded_object_md_t> > >, seastar::noncopyable_function<crimson::errorator<crimson::unthrowable_wrapper<std::error_code const&, crimson::ec<(std::errc)84> > >::_future<crimson::errorated_future_marker<std::unique_ptr<PGBackend::loaded_object_md_t, std::default_delete<PGBackend::loaded_object_md_t> > > > (seastar::future<std::map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, ceph::buffer::v15_2_0::list, std::less<void>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, ceph::buffer::v15_2_0::list> > > >&&)>, seastar::future<std::map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, ceph::buffer::v15_2_0::list, std::less<void>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, ceph::buffer::v15_2_0::list> > > >::then_wrapped_nrvo<crimson::errorator<crimson::unthrowable_wrapper<std::error_code const&, crimson::ec<(std::errc)84> > >::_future<crimson::errorated_future_marker<std::unique_ptr<PGBackend::loaded_object_md_t, std::default_delete<PGBackend::loaded_object_md_t> > > >, seastar::noncopyable_function<crimson::errorator<crimson::unthrowable_wrapper<std::error_code const&, crimson::ec<(std::errc)84> > >::_future<crimson::errorated_future_marker<std::unique_ptr<PGBackend::loaded_object_md_t, std::default_delete<PGBackend::loaded_object_md_t> > > > (seastar::future<std::map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, ceph::buffer::v15_2_0::list, std::less<void>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, ceph::buffer::v15_2_0::list> > > >&&)> >(seastar::noncopyable_function<crimson::errorator<crimson::unthrowable_wrapper<std::error_code const&, crimson::ec<(std::errc)84> > >::_future<crimson::errorated_future_marker<std::unique_ptr<PGBackend::loaded_object_md_t, std::default_delete<PGBackend::loaded_object_md_t> > > > (seastar::future<std::map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, ceph::buffer::v15_2_0::list, std::less<void>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, ceph::buffer::v15_2_0::list> > > >&&)>&&)::{lambda(seastar::internal::promise_base_with_type<std::unique_ptr<PGBackend::loaded_object_md_t, std::default_delete<PGBackend::loaded_object_md_t> > >&&, seastar::noncopyable_function<crimson::errorator<crimson::unthrowable_wrapper<std::error_code const&, crimson::ec<(std::errc)84> > >::_future<crimson::errorated_future_marker<std::unique_ptr<PGBackend::loaded_object_md_t, std::default_delete<PGBackend::loaded_object_md_t> > > > (seastar::future<std::map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, ceph::buffer::v15_2_0::list, std::less<void>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, ceph::buffer::v15_2_0::list> > > >&&)>&, seastar::future_state<std::map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, ceph::buffer::v15_2_0::list, std::less<void>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, ceph::buffer::v15_2_0::list> > > >&&)#1}, std::map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, ceph::buffer::v15_2_0::list, std::less<void>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, ceph::buffer::v15_2_0::list> > > >::run_and_dispose() in /usr/bin/ceph-osd
2021-06-15T09:25:07.512 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]:  7# 0x000055C99CFD195F in /usr/bin/ceph-osd
2021-06-15T09:25:07.513 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]:  8# 0x000055C99CFD6EA0 in /usr/bin/ceph-osd
2021-06-15T09:25:07.513 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]:  9# 0x000055C99D188F0B in /usr/bin/ceph-osd
2021-06-15T09:25:07.513 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]: 10# 0x000055C99CCE698A in /usr/bin/ceph-osd
2021-06-15T09:25:07.513 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]: 11# 0x000055C99CCF0AAE in /usr/bin/ceph-osd
2021-06-15T09:25:07.513 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]: 12# main in /usr/bin/ceph-osd
2021-06-15T09:25:07.513 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]: 13# __libc_start_main in /lib64/libc.so.6
2021-06-15T09:25:07.514 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]: 14# _start in /usr/bin/ceph-osd
2021-06-15T09:25:07.514 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:19 smithi100 conmon[54917]: Fault at location: 0x31dfff8000
2021-06-15T09:25:07.514 INFO:journalctl@ceph.osd.3.smithi100.stdout:Jun 15 09:24:20 smithi100 podman[55356]: 2021-06-15 09:24:20.230341885 +0000 UTC m=+0.072958807 container died a3ea2a1d0a176286b93b8f5b94458982b9038e70d09128fb55f53b92976f0c42 (image=quay.ceph.io/ceph-ci/ceph@sha256:13ae953e3f83ee011d784d6eb9126fdc692f5bb688fe7d918be61ca7a7282b3c, name=ceph-43579b90-cdba-11eb-8c13-001a4aab830c-osd.3)
```

The fix deals with the issue by wrapping the `bptrs` in `bufferlists`.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>