[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/netfilter/ip6_tables.c b/net/ipv6/netfilter/ip6_tables.c
index af1ec7b..70ef0d2 100644
--- a/net/ipv6/netfilter/ip6_tables.c
+++ b/net/ipv6/netfilter/ip6_tables.c
@@ -1879,11 +1879,11 @@
 
 	switch (cmd) {
 	case IP6T_SO_SET_REPLACE:
-		ret = compat_do_replace(sk->sk_net, user, len);
+		ret = compat_do_replace(sock_net(sk), user, len);
 		break;
 
 	case IP6T_SO_SET_ADD_COUNTERS:
-		ret = do_add_counters(sk->sk_net, user, len, 1);
+		ret = do_add_counters(sock_net(sk), user, len, 1);
 		break;
 
 	default:
@@ -1990,10 +1990,10 @@
 
 	switch (cmd) {
 	case IP6T_SO_GET_INFO:
-		ret = get_info(sk->sk_net, user, len, 1);
+		ret = get_info(sock_net(sk), user, len, 1);
 		break;
 	case IP6T_SO_GET_ENTRIES:
-		ret = compat_get_entries(sk->sk_net, user, len);
+		ret = compat_get_entries(sock_net(sk), user, len);
 		break;
 	default:
 		ret = do_ip6t_get_ctl(sk, cmd, user, len);
@@ -2012,11 +2012,11 @@
 
 	switch (cmd) {
 	case IP6T_SO_SET_REPLACE:
-		ret = do_replace(sk->sk_net, user, len);
+		ret = do_replace(sock_net(sk), user, len);
 		break;
 
 	case IP6T_SO_SET_ADD_COUNTERS:
-		ret = do_add_counters(sk->sk_net, user, len, 0);
+		ret = do_add_counters(sock_net(sk), user, len, 0);
 		break;
 
 	default:
@@ -2037,11 +2037,11 @@
 
 	switch (cmd) {
 	case IP6T_SO_GET_INFO:
-		ret = get_info(sk->sk_net, user, len, 0);
+		ret = get_info(sock_net(sk), user, len, 0);
 		break;
 
 	case IP6T_SO_GET_ENTRIES:
-		ret = get_entries(sk->sk_net, user, len);
+		ret = get_entries(sock_net(sk), user, len);
 		break;
 
 	case IP6T_SO_GET_REVISION_MATCH: