[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/icmp.c b/net/ipv6/icmp.c
index 5085766..63309d1 100644
--- a/net/ipv6/icmp.c
+++ b/net/ipv6/icmp.c
@@ -163,7 +163,7 @@
 				     struct flowi *fl)
 {
 	struct dst_entry *dst;
-	struct net *net = sk->sk_net;
+	struct net *net = sock_net(sk);
 	int res = 0;
 
 	/* Informational messages are not limited. */