This function does the work to update a checksum field as part of
remote checksum offload.
remcsum_adjust does the following:
1) Subtract out the calculated checksum from the beginning of the
packet (ptr arg) to the start offset.
2) Adjust the checksum field indicated by offset based on the modified
checksum value from above step.
3) Return the difference in the old checksum field value and the
new one. The caller will use this to update skb->csum and NAPI csum.
Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
7c967b224a