net: Fix GRE RX to use skb_transport_header for GRE header offset

GRE assumes that the GRE header is at skb_network_header +
ip_hrdlen(skb). It is more general to use skb_transport_header
and this allows the possbility of inserting additional header
between IP and GRE (which is what we will done in Generic UDP
Encapsulation for GRE).

Signed-off-by: Tom Herbert <therbert@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/gre_demux.c

@@ -98,15 +98,14 @@ EXPORT_SYMBOL_GPL(gre_build_header);
 static int parse_gre_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 			    bool *csum_err)
 {
-	unsigned int ip_hlen = ip_hdrlen(skb);
 	const struct gre_base_hdr *greh;
 	__be32 *options;
 	int hdr_len;
 
 	if (unlikely(!pskb_may_pull(skb, sizeof(struct gre_base_hdr))))
 		return -EINVAL;
 
-	greh = (struct gre_base_hdr *)(skb_network_header(skb) + ip_hlen);
+	greh = (struct gre_base_hdr *)skb_transport_header(skb);
 	if (unlikely(greh->flags & (GRE_VERSION | GRE_ROUTING)))
 		return -EINVAL;
 
@@ -116,7 +115,7 @@ static int parse_gre_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 	if (!pskb_may_pull(skb, hdr_len))
 		return -EINVAL;
 
-	greh = (struct gre_base_hdr *)(skb_network_header(skb) + ip_hlen);
+	greh = (struct gre_base_hdr *)skb_transport_header(skb);
 	tpi->proto = greh->protocol;
 
 	options = (__be32 *)(greh + 1);