ip6_gre: simplify gre header parsing in ip6gre_err

Same as ip_gre, use gre_parse_header to parse gre header in gre error
handler code.
Signed-off-by: NHaishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -427,35 +427,17 @@ static void ip6gre_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 		       u8 type, u8 code, int offset, __be32 info)
 {
 	struct net *net = dev_net(skb->dev);
-	const struct gre_base_hdr *greh;
 	const struct ipv6hdr *ipv6h;
-	int grehlen = sizeof(*greh);
+	struct tnl_ptk_info tpi;
 	struct ip6_tnl *t;
-	int key_off = 0;
-	__be16 flags;
-	__be32 key;
 
-	if (!pskb_may_pull(skb, offset + grehlen))
-		return;
-	greh = (const struct gre_base_hdr *)(skb->data + offset);
-	flags = greh->flags;
-	if (flags & (GRE_VERSION | GRE_ROUTING))
+	if (gre_parse_header(skb, &tpi, NULL, htons(ETH_P_IPV6),
+			     offset) < 0)
 		return;
-	if (flags & GRE_CSUM)
-		grehlen += 4;
-	if (flags & GRE_KEY) {
-		key_off = grehlen + offset;
-		grehlen += 4;
-	}
 
-	if (!pskb_may_pull(skb, offset + grehlen))
-		return;
 	ipv6h = (const struct ipv6hdr *)skb->data;
-	greh = (const struct gre_base_hdr *)(skb->data + offset);
-	key = key_off ? *(__be32 *)(skb->data + key_off) : 0;
-
 	t = ip6gre_tunnel_lookup(skb->dev, &ipv6h->daddr, &ipv6h->saddr,
-				 key, greh->protocol);
+				 tpi.key, tpi.proto);
 	if (!t)
 		return;