net: Convert proc_net_ns_ops

This patch starts to convert pernet_subsys, registered
before initcalls.

proc_net_ns_ops::proc_net_ns_init()/proc_net_ns_exit()
{un,}register pernet net->proc_net and ->proc_net_stat.

Constructors and destructors of another pernet_operations
are not interested in foreign net's proc_net and proc_net_stat.
Proc filesystem privitives are synchronized on proc_subdir_lock.

So, proc_net_ns_ops methods are able to be executed
in parallel with methods of any other pernet operations.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

fs/proc/proc_net.c

@@ -237,6 +237,7 @@ static __net_exit void proc_net_ns_exit(struct net *net)
 static struct pernet_operations __net_initdata proc_net_ns_ops = {
 	.init = proc_net_ns_init,
 	.exit = proc_net_ns_exit,
+	.async = true,
 };
 
 int __init proc_net_init(void)