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>

phermansson/dpdk,phermansson/dpdk,phermansson/dpdk,phermansson/dpdk