[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/af_inet.c b/net/ipv4/af_inet.c
index 06cfb0b..5882a13 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -464,7 +464,7 @@
 	if (addr_len < sizeof(struct sockaddr_in))
 		goto out;
 
-	chk_addr_ret = inet_addr_type(sk->sk_net, addr->sin_addr.s_addr);
+	chk_addr_ret = inet_addr_type(sock_net(sk), addr->sin_addr.s_addr);
 
 	/* Not specified by any standard per-se, however it breaks too
 	 * many applications when removed.  It is unfortunate since
@@ -802,7 +802,7 @@
 {
 	struct sock *sk = sock->sk;
 	int err = 0;
-	struct net *net = sk->sk_net;
+	struct net *net = sock_net(sk);
 
 	switch (cmd) {
 		case SIOCGSTAMP:
@@ -1132,7 +1132,7 @@
 	};
 
 	security_sk_classify_flow(sk, &fl);
-	err = ip_route_output_flow(sk->sk_net, &rt, &fl, sk, 0);
+	err = ip_route_output_flow(sock_net(sk), &rt, &fl, sk, 0);
 }
 	if (!err)
 		sk_setup_caps(sk, &rt->u.dst);