net: Add inet_proto_csum_replace_by_diff utility function

This function updates a checksum field value and skb->csum based on
a value which is the difference between the old and new checksum.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/checksum.h
Simple merge

diff --cc net/core/utils.c
Simple merge