rbd: add 'client_addr' sysfs rbd device attribute

Export client addr/nonce, so userspace can check if a image is being
blacklisted.

Signed-off-by: Mike Christie <mchristi@redhat.com>
[idryomov@gmail.com: ceph_client_addr(), endianess fix]
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>