l2tp: Remove redundant BUG_ON() check in l2tp_pernet

Passing NULL to l2tp_pernet causes a crash via BUG_ON.
Dereferencing net in net_generic() also has the same effect.
This patch removes the redundant BUG_ON check on the same parameter.
Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_core.c

@@ -122,8 +122,6 @@ static inline struct l2tp_tunnel *l2tp_tunnel(struct sock *sk)
 
 static inline struct l2tp_net *l2tp_pernet(const struct net *net)
 {
-	BUG_ON(!net);
-
 	return net_generic(net, l2tp_net_id);
 }