[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/ipv6/udp.c b/net/ipv6/udp.c
index 6683c04..db266ff 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -70,7 +70,7 @@
 	sk_for_each(sk, node, &udptable[hnum & (UDP_HTABLE_SIZE - 1)]) {
 		struct inet_sock *inet = inet_sk(sk);
 
-		if (sk->sk_net == net && sk->sk_hash == hnum &&
+		if (sock_net(sk) == net && sk->sk_hash == hnum &&
 				sk->sk_family == PF_INET6) {
 			struct ipv6_pinfo *np = inet6_sk(sk);
 			int score = 0;
@@ -323,7 +323,7 @@
 	sk_for_each_from(s, node) {
 		struct inet_sock *inet = inet_sk(s);
 
-		if (s->sk_net != sk->sk_net)
+		if (sock_net(s) != sock_net(sk))
 			continue;
 
 		if (s->sk_hash == num && s->sk_family == PF_INET6) {