net: Convert proto_gre_net_ops

These pernet_operations register and unregister sysctl.
nf_conntrack_l4proto_gre4->init_net is simple memory
initializer. Also, exit method removes gre keymap_list,
which is per-net. This looks safe to be executed
in parallel with other pernet_operations.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netfilter/nf_conntrack_proto_gre.c

@@ -406,6 +406,7 @@ static struct pernet_operations proto_gre_net_ops = {
 	.exit = proto_gre_net_exit,
 	.id   = &proto_gre_net_id,
 	.size = sizeof(struct netns_proto_gre),
+	.async = true,
 };
 
 static int __init nf_ct_proto_gre_init(void)