net: Convert ipv6_sysctl_net_ops

These pernet_operations create and destroy sysctl tables.
They are not touched by another net pernet_operations.
So, it's possible to execute them in parallel with others.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/sysctl_net_ipv6.c

@@ -251,6 +251,7 @@ static void __net_exit ipv6_sysctl_net_exit(struct net *net)
 static struct pernet_operations ipv6_sysctl_net_ops = {
 	.init = ipv6_sysctl_net_init,
 	.exit = ipv6_sysctl_net_exit,
+	.async = true,
 };
 
 static struct ctl_table_header *ip6_header;