mds: add the revoking caps back to _revokes list

mds: add the revoking caps back to _revokes list

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>
(cherry picked from commit 0454c449132abd1499d671990d680bd0794abc5a)