net: fix unneeded replacement of TCP checksum 0

net: fix unneeded replacement of TCP checksum 0

Per RFC768:
If the computed checksum is zero, it is transmitted as all ones.
An all zero transmitted checksum value means that the transmitter
generated no checksum.

RFC793 for TCP has no such special treatment for the checksum of zero.

Fixes: 6006818cfb26 ("net: new checksum functions")
Cc: stable@dpdk.org
Signed-off-by: Hongzhi Guo <guohongzhi1@huawei.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Morten Brørup <mb@smartsharesystems.com>