When revoking caps from clients and if the clients could release
some of the caps references and the clients still could send cap
update request back to MDS, while the confirm_receipt() will clear
the _revokes list anyway.
But this cap will still be kept in revoking_caps list.
At the same time add one debug log when revocation is not totally
finished.
Fixes: https://tracker.ceph.com/issues/57244 Signed-off-by: Xiubo Li <xiubli@redhat.com>