rbd: reacquire lock should update lock owner client id

Otherwise, future operations on this RBD using exclusive-lock are
going to require the lock from a non-existent client id.

Cc: stable@vger.kernel.org
Fixes: 14bb211d

 ("rbd: support updating the lock cookie without releasing the lock")
Link: http://tracker.ceph.com/issues/19929
Signed-off-by: Florian Margaine <florian@platform.sh>
[idryomov@gmail.com: rbd_set_owner_cid() call, __rbd_lock() helper]
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>