net: Convert nfs4_dns_resolver_ops

These pernet_operations look similar to rpcsec_gss_net_ops,
they just create and destroy another cache. Also they create
and destroy directory. So, they also look safe to be async.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Anna Schumaker <Anna.Schumaker@netapp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

fs/nfs/dns_resolve.c

@@ -410,6 +410,7 @@ static void nfs4_dns_net_exit(struct net *net)
 static struct pernet_operations nfs4_dns_resolver_ops = {
 	.init = nfs4_dns_net_init,
 	.exit = nfs4_dns_net_exit,
+	.async = true,
 };
 
 static int rpc_pipefs_event(struct notifier_block *nb, unsigned long event,