net: Convert cttimeout_ops

These pernet_operations also look closed in themself.
Exit method touch only per-net structures, so it's
safe to execute them for several net namespaces in parallel.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netfilter/nfnetlink_cttimeout.c

@@ -586,6 +586,7 @@ static void __net_exit cttimeout_net_exit(struct net *net)
 static struct pernet_operations cttimeout_ops = {
 	.init	= cttimeout_net_init,
 	.exit	= cttimeout_net_exit,
+	.async	= true,
 };
 
 static int __init cttimeout_init(void)