ceph: use fixed endian encoding for ceph_entity_addr

We exchange struct ceph_entity_addr over the wire and store it on disk.
The sockaddr_storage.ss_family field, however, is host endianness.  So,
fix ss_family endianness to big endian when sending/receiving over the
wire.

Signed-off-by: Sage Weil <sage@newdream.net>