rbd: support updating the lock cookie without releasing the lock

As we no longer release the lock before potentially raising BLACKLISTED
in rbd_reregister_watch(), the "either locked or blacklisted" assert in
rbd_queue_workfn() needs to go: we can be both locked and blacklisted
at that point now.

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