libceph: fix watch_item_t decoding to use ceph_decode_entity_addr

While we're in there, let's also fix up the decoder to do proper
bounds checking.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>