Commit d8394185304c "[net] Store checksum result for offloaded GSO checksums"
introduces skb_checksum_start() helper, which is the utilised in 3eb624d908a0
"[netdrv] ixgbe/ixgbevf: Don't use lco_csum to compute IPv4 checksum" (upstream
c54cdc316dbd) and 5517f5c1beae "[netdrv] ixgbe/ixgbevf: Add support for GSO
partial" (upstream b83e30104bd9).

Backport changes: add implementation for skb_checksum_start() from "[net] Store
checksum result for offloaded GSO checksums" to ixgbe_backport_compat.h.
Index: src/drivers/net/ethernet/intel/ixgbe/ixgbe_backport_compat.h
===================================================================
--- src.orig/drivers/net/ethernet/intel/ixgbe/ixgbe_backport_compat.h	2017-03-10 18:11:19.807312186 +0100
+++ src/drivers/net/ethernet/intel/ixgbe/ixgbe_backport_compat.h	2017-03-10 18:11:39.660099187 +0100
@@ -2,6 +2,7 @@
 #define _RH_IXGBE_BACKPORT_COMPAT_H_
 
 #include <linux/netdevice.h>
+#include <linux/skbuff.h>
 #include <net/udp_tunnel.h>
 
 /* legacy drivers only, netdev_start_xmit() sets txq->trans_start */
@@ -47,4 +48,10 @@
 	call_netdevice_notifiers(NETDEV_OFFLOAD_PUSH_GENEVE, dev);
 }
 
+
+static inline unsigned char *skb_checksum_start(const struct sk_buff *skb)
+{
+	return skb->head + skb->csum_start;
+}
+
 #endif /* #ifndef _RH_IXGBE_BACKPORT_COMPAT_H_ */