net: Convert ipv4_net_ops

These pernet_operations register and unregister bunch
of nf_conntrack_l4proto. Exit method unregisters related
sysctl, init method calls init_net and get_net_proto.
The whole builtin_l4proto4 array has pretty simple
init_net and get_net_proto methods. The first one register
sysctl table, the second one is just RO memory dereference.
So, these pernet_operations are safe 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_conntrack_l3proto_ipv4.c

@@ -399,6 +399,7 @@ static struct pernet_operations ipv4_net_ops = {
 	.exit = ipv4_net_exit,
 	.id = &conntrack4_net_id,
 	.size = sizeof(struct conntrack4_net),
+	.async = true,
 };
 
 static int __init nf_conntrack_l3proto_ipv4_init(void)