rbd: ignore unlock errors

Currently the lock_state is set to UNLOCKED (preventing further I/O),
but RELEASED_LOCK notification isn't sent.  Be consistent with userspace
and treat ceph_cls_unlock() errors as the image is unlocked.

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