net: add skb functions to process remote checksum offload

This patch adds skb_remcsum_process and skb_gro_remcsum_process to
perform the appropriate adjustments to the skb when receiving
remote checksum offload.

Updated vxlan and gue to use these functions.

Tested: Ran TCP_RR and TCP_STREAM netperf for VXLAN and GUE, did
not see any change in performance.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 16251e9..1347ac5 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -2318,6 +2318,21 @@
 					   compute_pseudo(skb, proto));	\
 } while (0)
 
+static inline void skb_gro_remcsum_process(struct sk_buff *skb, void *ptr,
+					   int start, int offset)
+{
+	__wsum delta;
+
+	BUG_ON(!NAPI_GRO_CB(skb)->csum_valid);
+
+	delta = remcsum_adjust(ptr, NAPI_GRO_CB(skb)->csum, start, offset);
+
+	/* Adjust skb->csum since we changed the packet */
+	skb->csum = csum_add(skb->csum, delta);
+	NAPI_GRO_CB(skb)->csum = csum_add(NAPI_GRO_CB(skb)->csum, delta);
+}
+
+
 static inline int dev_hard_header(struct sk_buff *skb, struct net_device *dev,
 				  unsigned short type,
 				  const void *daddr, const void *saddr,