net: Convert arptable_filter_net_ops

These pernet_operations unregister net::ipv4::arptable_filter.
Another net/pernet_operations do not send arp packets to foreign
net namespaces. So, we mark them async.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/arptable_filter.c

@@ -65,6 +65,7 @@ static void __net_exit arptable_filter_net_exit(struct net *net)
 
 static struct pernet_operations arptable_filter_net_ops = {
 	.exit = arptable_filter_net_exit,
+	.async = true,
 };
 
 static int __init arptable_filter_init(void)