app/testpmd: fix IPv4 checksum

app/testpmd: fix IPv4 checksum

UDP protocol reserves 0 checksum value for special purposes.
Other protocols, like IPv4, TCP and SCTP must calculate checksum value
in software or offload checksum calculation to hardware.

If IPv4 TX checksum offload was off and header checksum was set to 0,
testpmd csum engine did not calculate checksum value for IPv4, TCP and
SCTP.

The patch always calculates IPv4, TCP and SCTP TX checksums if it is
not offloaded.

Bugzilla ID: 768
Fixes: b2a9e4a855d0 ("app/testpmd: fix Tx checksum calculation for tunnel")
Cc: stable@dpdk.org
Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>