app/testpmd: use Tx preparation in checksum engine

app/testpmd: use Tx preparation in checksum engine

Since all current drivers supports Tx preparation API, it is used
in csum forwarding engine by default for all drivers.

Adding additional step to the csum engine costs about 3-4% of performance
drop, on my setup with ixgbe driver. It's caused mostly by the need
of reaccessing and modification of packet data.

Signed-off-by: Tomasz Kulasek <tomaszx.kulasek@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>