ceph: force sending a cap update msg back to MDS for revoke op

If a client sends out a cap update dropping caps with the prior 'seq'
just before an incoming cap revoke request, then the client may drop
the revoke because it believes it's already released the requested
capabilities.

This causes the MDS to wait indefinitely for the client to respond
to the revoke. It's therefore always a good idea to ack the cap
revoke request with the bumped up 'seq'.

Currently if the cap->issued equals to the newcaps the check_caps()
will do nothing, we should force flush the caps.

Cc: stable@vger.kernel.org
Link: https://tracker.ceph.com/issues/61782


Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>