app/testpmd: support NVGRE in Tx checksum offload
app/testpmd: support NVGRE in Tx checksum offload

Enhance csum fwd engine based on current TX checksum framework in order
to test TX Checksum offload for NVGRE packet.

It includes:
 - IPv4 and IPv6 packet
 - outer L3, inner L3 and L4 checksum offload for Tx side.

Signed-off-by: Jijiang Liu <10b95deab7df654d0c21c8c98b54c9a1ecf2a5c0@intel.com>
Signed-off-by: Declan Doherty <b4388acd850cd8caad5000a112795063f04807ac@intel.com>

john-mcnamara-intel/dpdk,msune/dpdk,mixja/dpdk,tsphillips/dpdk-fork,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,venkynv/dpdk-mirror,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,tsphillips/dpdk-fork,tsphillips/dpdk-fork,mixja/dpdk,venkynv/dpdk-mirror,msune/dpdk,mixja/dpdk,john-mcnamara-intel/dpdk,msune/dpdk,mixja/dpdk,venkynv/dpdk-mirror,msune/dpdk