Checksum calculation on data including its own checksum (as is the case with
IP headers) should give 0xffff before bit flipping. Current code works well
for the correct checksum case, but fails to identify errors.
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>