gre: Use inner mac length when computing tunnel length

Currently, skb_inner_network_header is used but this does not account
for Ethernet header for ETH_P_TEB. Use skb_inner_mac_header which
handles TEB and also should work with IP encapsulation in which case
inner mac and inner network headers are the same.

Tested: Ran TCP_STREAM over GRE, worked as expected.
Signed-off-by: Tom Herbert <therbert@google.com>
Acked-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/gre_offload.c

@@ -47,7 +47,7 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 
 	greh = (struct gre_base_hdr *)skb_transport_header(skb);
 
-	ghl = skb_inner_network_header(skb) - skb_transport_header(skb);
+	ghl = skb_inner_mac_header(skb) - skb_transport_header(skb);
 	if (unlikely(ghl < sizeof(*greh)))
 		goto out;