net: Convert nf_tables_net_ops

These pernet_operations looks nicely separated per-net.
Exit method unregisters net's nf tables objects.
We allow them be executed in parallel.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netfilter/nf_tables_api.c

@@ -6596,6 +6596,7 @@ static void __net_exit nf_tables_exit_net(struct net *net)
 static struct pernet_operations nf_tables_net_ops = {
 	.init	= nf_tables_init_net,
 	.exit	= nf_tables_exit_net,
+	.async	= true,
 };
 
 static int __init nf_tables_module_init(void)