[NET] NETNS: Omit sock->sk_net without CONFIG_NET_NS.

Introduce per-sock inlines: sock_net(), sock_net_set()
and per-inet_timewait_sock inlines: twsk_net(), twsk_net_set().
Without CONFIG_NET_NS, no namespace other than &init_net exists.
Let's explicitly define them to help compiler optimizations.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
diff --git a/net/tipc/socket.c b/net/tipc/socket.c
index 3220d5c..ae45df0 100644
--- a/net/tipc/socket.c
+++ b/net/tipc/socket.c
@@ -1375,7 +1375,7 @@
 	}
 	buf = skb_peek(&sock->sk->sk_receive_queue);
 
-	res = tipc_create(sock->sk->sk_net, newsock, 0);
+	res = tipc_create(sock_net(sock->sk), newsock, 0);
 	if (!res) {
 		struct tipc_sock *new_tsock = tipc_sk(newsock->sk);
 		struct tipc_portid id;