l2tp: test for ethernet header in l2tp_eth_dev_recv()

close https://bugzilla.kernel.org/show_bug.cgi?id=16529

Before calling dev_forward_skb(), we should make sure skb head contains
at least an ethernet header, even if length included in upper layer said
so. Use pskb_may_pull() to make sure this ethernet header is present in
skb head.
Reported-by: NThomas Heil <heil@terminal-consulting.de>
Reported-by: NIan Campbell <Ian.Campbell@eu.citrix.com>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/l2tp/l2tp_eth.c

@@ -132,7 +132,7 @@ static void l2tp_eth_dev_recv(struct l2tp_session *session, struct sk_buff *skb,
 		printk("\n");
 	}
 
-	if (data_len < ETH_HLEN)
+	if (!pskb_may_pull(skb, sizeof(ETH_HLEN)))
 		goto error;
 
 	secpath_reset(skb);