[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/x25/af_x25.c

@@ -549,7 +549,7 @@ static struct sock *x25_make_new(struct sock *osk)
 	if (osk->sk_type != SOCK_SEQPACKET)
 		goto out;
 
-	if ((sk = x25_alloc_socket(osk->sk_net)) == NULL)
+	if ((sk = x25_alloc_socket(sock_net(osk))) == NULL)
 		goto out;
 
 	x25 = x25_sk(sk);