[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/fib6_rules.c b/net/ipv6/fib6_rules.c
index e7a7fe2..cac5807 100644
--- a/net/ipv6/fib6_rules.c
+++ b/net/ipv6/fib6_rules.c
@@ -154,7 +154,7 @@
 			       struct nlattr **tb)
 {
 	int err = -EINVAL;
-	struct net *net = skb->sk->sk_net;
+	struct net *net = sock_net(skb->sk);
 	struct fib6_rule *rule6 = (struct fib6_rule *) rule;
 
 	if (rule->action == FR_ACT_TO_TBL) {