net: Convert defrag4_net_ops

These pernet_operations only unregister nf hooks.
So, they are able to be marked as async.

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

net/ipv4/netfilter/nf_defrag_ipv4.c

@@ -118,6 +118,7 @@ static void __net_exit defrag4_net_exit(struct net *net)
 
 static struct pernet_operations defrag4_net_ops = {
 	.exit = defrag4_net_exit,
+	.async = true,
 };
 
 static int __init nf_defrag_init(void)