SUNRPC: fixup for namespace changes

Fixes this build error when CONFIG_NET_NS is not set:

net/sunrpc/svcsock.c: In function 'svc_setup_socket':
net/sunrpc/svcsock.c:1412:40: error: 'struct sock_common' has no member named 'skc_net'

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/svcsock.c

@@ -1409,7 +1409,7 @@ static struct svc_sock *svc_setup_socket(struct svc_serv *serv,
 
 	/* Register socket with portmapper */
 	if (*errp >= 0 && pmap_register)
-		*errp = svc_register(serv, sock->sk->sk_net, inet->sk_family,
+		*errp = svc_register(serv, sock_net(sock->sk), inet->sk_family,
 				     inet->sk_protocol,
 				     ntohs(inet_sk(inet)->inet_sport));