netns: export netns list

Conntrack code will use it for
a) removing expectations and helpers when corresponding module is removed, and
b) removing conntracks when L3 protocol conntrack module is removed.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>

net/core/net_namespace.c

@@ -18,6 +18,7 @@ static struct list_head *first_device = &pernet_list;
 static DEFINE_MUTEX(net_mutex);
 
 LIST_HEAD(net_namespace_list);
+EXPORT_SYMBOL_GPL(net_namespace_list);
 
 struct net init_net;
 EXPORT_SYMBOL(init_net);