examples/tep_term: fix L4 length
examples/tep_term: fix L4 length

l4_len is not fixed, althrough mostly it is a fixed value,
but when guest using iperf to do some tests, the l4_len
will have another 12 bytes optional fields.

Fixes: 2bb43bd4350a ("examples/tep_term: add TSO offload configuration")

Signed-off-by: Michael Qiu <164eb618f80262e3b92d2dccf1e320041dfc8caa@chinac.com>
Acked-by: Jianfeng Tan <4339d9ba3a87a0d192e405945005ff86667f7b0c@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk