ipv6: fix a potential use after free in sit.c

pskb_may_pull() maybe change skb->data and make iph pointer oboslete,
fix it by geting ip header length directly.

Fixes: ca15a078 (sit: generate icmpv6 error when receiving icmpv4 error)
Cc: Oussama Ghorbel <ghorbel@pivasoftware.com>
Signed-off-by: NLi RongQing <roy.qing.li@gmail.com>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -485,11 +485,11 @@ static void ipip6_tunnel_uninit(struct net_device *dev)
  */
 static int ipip6_err_gen_icmpv6_unreach(struct sk_buff *skb)
 {
-	const struct iphdr *iph = (const struct iphdr *) skb->data;
+	int ihl = ((const struct iphdr *)skb->data)->ihl*4;
 	struct rt6_info *rt;
 	struct sk_buff *skb2;
 
-	if (!pskb_may_pull(skb, iph->ihl * 4 + sizeof(struct ipv6hdr) + 8))
+	if (!pskb_may_pull(skb, ihl + sizeof(struct ipv6hdr) + 8))
 		return 1;
 
 	skb2 = skb_clone(skb, GFP_ATOMIC);
@@ -498,7 +498,7 @@ static int ipip6_err_gen_icmpv6_unreach(struct sk_buff *skb)
 		return 1;
 
 	skb_dst_drop(skb2);
-	skb_pull(skb2, iph->ihl * 4);
+	skb_pull(skb2, ihl);
 	skb_reset_network_header(skb2);
 
 	rt = rt6_lookup(dev_net(skb->dev), &ipv6_hdr(skb2)->saddr, NULL, 0, 0);