net: Convert defrag6_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/ipv6/netfilter/nf_defrag_ipv6_hooks.c

@@ -103,6 +103,7 @@ static void __net_exit defrag6_net_exit(struct net *net)
 
 static struct pernet_operations defrag6_net_ops = {
 	.exit = defrag6_net_exit,
+	.async = true,
 };
 
 static int __init nf_defrag_init(void)