ip_gre: fix a possible crash in ipgre_err()

Another fix needed in ipgre_err(), as parse_gre_header() might change
skb->head.

Bug added in commit c54419321455 (GRE: Refactor GRE tunneling code.)

Signed-off-by: Eric Dumazet <[e-mail]>
Cc: Pravin B Shelar <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/ip_gre.c

···
 	   */
 	struct net *net = dev_net(skb->dev);
 	struct ip_tunnel_net *itn;
-	const struct iphdr *iph = (const struct iphdr *)skb->data;
+	const struct iphdr *iph;
 	const int type = icmp_hdr(skb)->type;
 	const int code = icmp_hdr(skb)->code;
 	struct ip_tunnel *t;
···
 	else
 		itn = net_generic(net, ipgre_net_id);
 
+	iph = (const struct iphdr *)skb->data;
 	t = ip_tunnel_lookup(itn, skb->dev->ifindex, tpi.flags,
 			     iph->daddr, iph->saddr, tpi.key);