l2tp: fix a typo in l2tp_eth_dev_recv()

[ Upstream commit c0cc88a7627c333de50b07b7c60b1d49d9d2e6cc ]

While investigating l2tp bug, I hit a bug in eth_type_trans(),
because not enough bytes were pulled in skb head.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>