app/testpmd: fix TSO when using outer checksum offloads
app/testpmd: fix TSO when using outer checksum offloads

The l4_len has also to be copied in mbuf in case we are offloading outer
IP checksum. Currently, TSO + outer checksum is not supported by any
driver but it will soon be supported by i40e.

Reported-by: Jijiang Liu <10b95deab7df654d0c21c8c98b54c9a1ecf2a5c0@intel.com>
Signed-off-by: Olivier Matz <dfcc1510895413197abb4336e39bb0a3906cba71@6wind.com>

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