pppol2tp: calls unregister_pernet_gen_device() at unload time

Failure to call unregister_pernet_gen_device() can exhaust memory
if module is loaded/unloaded many times.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/pppol2tp.c

@@ -2680,6 +2680,7 @@ static int __init pppol2tp_init(void)
 static void __exit pppol2tp_exit(void)
 {
 	unregister_pppox_proto(PX_PROTO_OL2TP);
+	unregister_pernet_gen_device(pppol2tp_net_id, &pppol2tp_net_ops);
 	proto_unregister(&pppol2tp_sk_proto);
 }