rbd: don't mess with a page vector in rbd_notify_op_lock()

rbd_notify_op_lock() isn't interested in a notify reply.  Instead of
accepting that page vector just to free it, have watch-notify code take
care of it.

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