os/bluestore/bluefs: Fix race condition between truncate() and unlink()

os/bluestore/bluefs: Fix race condition between truncate() and unlink()

It was possible for unlink() to interrupt ongoing truncate().
As the result, unlink() finishes properly, but truncate() is not aware
of it and does:
1) updates file that is already removed
2) releases same allocations again

Now fixed by checking if file is deleted under FILE lock.

https://tracker.ceph.com/issues/70747

Signed-off-by: Adam Kupczyk <akupczyk@ibm.com>