l2tp: fix a typo in l2tp_eth_dev_recv()

While investigating l2tp bug, I hit a bug in eth_type_trans(),
because not enough bytes were pulled in skb head.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/l2tp/l2tp_eth.c

@@ -153,7 +153,7 @@ static void l2tp_eth_dev_recv(struct l2tp_session *session, struct sk_buff *skb,
 		print_hex_dump_bytes("", DUMP_PREFIX_OFFSET, skb->data, length);
 	}
 
-	if (!pskb_may_pull(skb, sizeof(ETH_HLEN)))
+	if (!pskb_may_pull(skb, ETH_HLEN))
 		goto error;
 
 	secpath_reset(skb);