rbd: store lock cookie

In preparation for supporting set_cookie method (or rather set_cookie
fallback for older OSDs), store the lock cookie on lock and use it on
unlock instead of recalculating from rbd_dev->watch_cookie.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>