librbd/mirror: detect trashed snapshots in UnlinkPeerRequest

librbd/mirror: detect trashed snapshots in UnlinkPeerRequest

If two instances of UnlinkPeerRequest race with each other (e.g. due
to rbd-mirror daemon unlinking from a previous mirror snapshot and the
user taking another mirror snapshot at same time), the snapshot that
UnlinkPeerRequest was created for may be in the process of being removed
(which may mean trashed by SnapshotRemoveRequest::trash_snap()) or fully
removed by the time unlink_peer() grabs the image lock.  Because trashed
snapshots weren't handled explicitly, UnlinkPeerRequest could spuriously
fail with EINVAL ("not mirror snapshot" case) instead of the expected
ENOENT ("missing snapshot" case).  This in turn could lead to spurious
ImageReplayer failures with it stopping prematurely.

Fixes: https://tracker.ceph.com/issues/68279
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>