l2tp: add missing .owner to struct pppox_proto

Add missing .owner of struct pppox_proto. This prevents the
module from being removed from underneath its users.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ppp.c

@@ -1793,7 +1793,8 @@ static const struct proto_ops pppol2tp_ops = {
 
 static const struct pppox_proto pppol2tp_proto = {
 	.create		= pppol2tp_create,
-	.ioctl		= pppol2tp_ioctl
+	.ioctl		= pppol2tp_ioctl,
+	.owner		= THIS_MODULE,
 };
 
 #ifdef CONFIG_L2TP_V3