net: gre: use icmp_hdr() to get inner ip header

When dealing with icmp messages, the skb->data points the
ip header that triggered the sending of the icmp message.

In gre_cisco_err(), the parse_gre_header() is called, and the
iptunnel_pull_header() is called to pull the skb at the end of
the parse_gre_header(), so the skb->data doesn't point the
inner ip header.

Unfortunately, the ipgre_err still needs those ip addresses in
inner ip header to look up tunnel by ip_tunnel_lookup().

So just use icmp_hdr() to get inner ip header instead of skb->data.
Signed-off-by: Duan Jiong <duanj.fnst@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -178,7 +178,7 @@ static int ipgre_err(struct sk_buff *skb, u32 info,
 	else
 		itn = net_generic(net, ipgre_net_id);
 
-	iph = (const struct iphdr *)skb->data;
+	iph = (const struct iphdr *)(icmp_hdr(skb) + 1);
 	t = ip_tunnel_lookup(itn, skb->dev->ifindex, tpi->flags,
 			     iph->daddr, iph->saddr, tpi->key);