app/testpmd: do not change IP addrs in checksum engine

app/testpmd: do not change IP addrs in checksum engine

The csum forward engine was updated to change the IP addresses in the
packet data in
commit 51f694dd40f5 ("app/testpmd: rework checksum forward engine")

This was done to ensure that the checksum is correctly reprocessed when
using hardware checksum offload. But the functions
process_inner_cksums() and process_outer_cksums() already reset the
checksum field to 0, so this is not necessary.

Moreover, this makes the engine more complex than needed, and prevents
to easily use it to forward traffic (like iperf) as it modifies the
packets.

This patch drops this behavior.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>