netlink: add nlk->netlink_bind hook for module auto-loading

This patch adds a hook in the binding path of netlink.

This is used by ctnetlink to allow module autoloading for the case
in which one user executes:

 conntrack -E

So far, this resulted in nfnetlink loaded, but not
nf_conntrack_netlink.

I have received in the past many complains on this behaviour.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 6085e49..f74dd13 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -179,6 +179,7 @@
 	unsigned int	groups;
 	void		(*input)(struct sk_buff *skb);
 	struct mutex	*cb_mutex;
+	void		(*bind)(int group);
 };
 
 extern struct sock *netlink_kernel_create(struct net *net, int unit,