crimson/os/alienstore: fix nullptr deref in OnCommit::finish().

crimson/os/alienstore: fix nullptr deref in OnCommit::finish().

`seastar::engine()` is available only for Seastar's threads;
it shouldn't be called outside of a reactor thread.
Unfortunately, this assumption is violated in `AlienStore`
where `OnCommit::finish()`, executed from a finisher thread
of `BlueStore`, calls `alien()` on `seastar::engine()`.
The net effect are crashes like the following one:

```
INFO  2021-09-22 14:26:33,214 [shard 0] osd - operator() writing superblock cluster_fsid 1d8f7908-2ebf-4a91-ae70-f445668c126b osd_fsid 4da9fe9a-1da5-4ea9-aa79-a1178165ede5         [381/1839]
Segmentation fault.
Backtrace:
 0# print_backtrace(std::basic_string_view<char, std::char_traits<char> >) at /home/rzarzynski/ceph1/build/../src/crimson/common/fatal_signal.cc:80
 1# FatalSignal::signaled(int, siginfo_t const&) at /opt/rh/gcc-toolset-9/root/usr/include/c++/9/ostream:570
 2# FatalSignal::install_oneshot_signal_handler<11>()::{lambda(int, siginfo_t*, void*)#1}::_FUN(int, siginfo_t*, void*) at /home/rzarzynski/ceph1/build/../src/crimson/common/fatal_signal.cc:
62
 3# 0x00007F16BBA13B30 in /lib64/libpthread.so.0
 4# (anonymous namespace)::OnCommit::finish(int) at /home/rzarzynski/ceph1/build/../src/crimson/os/alienstore/alien_store.cc:53
 5# Context::complete(int) at /home/rzarzynski/ceph1/build/../src/include/Context.h:100
 6# Finisher::finisher_thread_entry() at /home/rzarzynski/ceph1/build/../src/common/Finisher.cc:65
 7# 0x00007F16BBA0915A in /lib64/libpthread.so.0
 8# clone in /lib64/libc.so.6
Dump of siginfo:
  ...
  si_addr: 0x10
```

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