net: Convert xfrm_user_net_ops

These pernet_operations create and destroy net::xfrm::nlsk
socket of NETLINK_XFRM. There is only entry point, where
it's dereferenced, it's xfrm_user_rcv_msg(). There is no
in-kernel senders to this socket.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/xfrm/xfrm_user.c

@@ -3258,6 +3258,7 @@ static void __net_exit xfrm_user_net_exit(struct list_head *net_exit_list)
 static struct pernet_operations xfrm_user_net_ops = {
 	.init	    = xfrm_user_net_init,
 	.exit_batch = xfrm_user_net_exit,
+	.async	    = true,
 };
 
 static int __init xfrm_user_init(void)