[NETNS]: Introduce a netns_core structure.

There's already some stuff on the struct net, that should better
be folded into netns_core structure. I'm making the per-proto inuse 
counter be per-net also, which is also a candidate for this, so 
introduce this structure and populate it a bit.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/socket.c b/net/socket.c
index 79e5382..9b5c917 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1375,7 +1375,7 @@
 
 	sock = sockfd_lookup_light(fd, &err, &fput_needed);
 	if (sock) {
-		somaxconn = sock_net(sock->sk)->sysctl_somaxconn;
+		somaxconn = sock_net(sock->sk)->core.sysctl_somaxconn;
 		if ((unsigned)backlog > somaxconn)
 			backlog = somaxconn;