[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>

net/atm/svc.c

@@ -326,7 +326,7 @@ static int svc_accept(struct socket *sock,struct socket *newsock,int flags)
 
 	lock_sock(sk);
 
-	error = svc_create(sk->sk_net, newsock,0);
+	error = svc_create(sock_net(sk), newsock,0);
 	if (error)
 		goto out;