samples: pktgen: add UDP tx checksum support

Introduce k parameter in pktgen samples in order to toggle UDP tx
checksum

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Link: https://lore.kernel.org/r/cf16417902062c6ea2fd3c79e00510e36a40c31a.1622210713.git.lorenzo@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>