SUNRPC: svc_setup_socket() gets protocol family from socket

Since the sv_family field is going away, modify svc_setup_socket() to
extract the protocol family from the passed-in socket instead of from
the passed-in svc_serv struct.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>