[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/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 913266c..0834926 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -351,7 +351,7 @@
 			 * itself out.
 			 */
 			security_sk_classify_flow(sk, &fl);
-			if (ip_route_output_flow(sk->sk_net, &rt, &fl, sk, 0))
+			if (ip_route_output_flow(sock_net(sk), &rt, &fl, sk, 0))
 				goto no_route;
 		}
 		sk_setup_caps(sk, &rt->u.dst);
@@ -1382,7 +1382,7 @@
 						 .dport = tcp_hdr(skb)->source } },
 				    .proto = sk->sk_protocol };
 		security_skb_classify_flow(skb, &fl);
-		if (ip_route_output_key(sk->sk_net, &rt, &fl))
+		if (ip_route_output_key(sock_net(sk), &rt, &fl))
 			return;
 	}