GRE: enable offloads for GRE

To get offloads to work with Generic Routing Encapsulation (GRE), the
outer transport header has to be reset after skb_push is done. This
patch has the support for this fix and hence GRE offloading.
Signed-off-by: Amritha Nambiar <amritha.nambiar@intel.com>
Signed-off-by: Joseph Gasparakis <joseph.gasparakis@intel.com>
Tested-By: Jim Young <jamesx.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/gre_demux.c

@@ -68,6 +68,7 @@ void gre_build_header(struct sk_buff *skb, const struct tnl_ptk_info *tpi,
 
 	skb_push(skb, hdr_len);
 
+	skb_reset_transport_header(skb);
 	greh = (struct gre_base_hdr *)skb->data;
 	greh->flags = tnl_flags_to_gre_flags(tpi->flags);
 	greh->protocol = tpi->proto;