l2tp: add missing .owner to struct pppox_proto

[ Upstream commit e1558a93b61962710733dc8c11a2bc765607f1cd ]

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>