net: fix underflow for checksum of invalid IPv4 packets

net: fix underflow for checksum of invalid IPv4 packets

If we receive a packet with an invalid IP header, where the total packet
length is reported as less than the IP header length, we would end up
getting an underflow in the length subtraction.

This could cause us to checksum e.g. 4GB of data in the case where the
result of the subtraction was -1.

We fix this by having the function return 0 - an invalid sum - when
the length is less than the header length.

Fixes: af75078fece3 ("first public release")
Fixes: 6006818cfb26 ("net: new checksum functions")
Cc: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>