[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/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 086deff..323c7e0 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1218,7 +1218,7 @@
 	if (req)
 		return tcp_check_req(sk, skb, req, prev);
 
-	nsk = __inet6_lookup_established(sk->sk_net, &tcp_hashinfo,
+	nsk = __inet6_lookup_established(sock_net(sk), &tcp_hashinfo,
 			&ipv6_hdr(skb)->saddr, th->source,
 			&ipv6_hdr(skb)->daddr, ntohs(th->dest), inet6_iif(skb));