pppoe.c: Fix kernel panic caused by __pppoe_xmit

__pppoe_xmit function return value was invalid resulting in
additional call to kfree_skb on already freed skb. This resulted in
memory corruption and consequent kernel panic after PPPoE peer
terminated the link.

This fixes commit 55c95e73.

Reported-by: Gorik Van Steenberge <gvs@zemos.net>
Reported-by: Daniel Kenzelmann <kernel.bugzilla@kenzelmann.dyndns.info>
Reported-by: Denys Fedoryshchenko <nuclearcat@nuclearcat.com>
Reported-by: Pawel Staszewski <pstaszewski@artcom.pl>
Diagnosed-by: Andrej Ota <andrej@ota.si>
Diagnosed-by: Eric Dumazet <eric.dumazet@gmail.com>
Tested-by: Denys Fedoryshchenko <nuclearcat@nuclearcat.com>
Tested-by: Pawel Staszewski <pstaszewski@artcom.pl>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: Andrej Ota <andrej@ota.si>
Signed-off-by: David S. Miller <davem@davemloft.net>