app/testpmd: fix GTP PSC extension header length
app/testpmd: fix GTP PSC extension header length

Current implementation for raw encap sets the length to be in bytes,
but, GTP 'extension' header length is an 8-bit field in 4-octet units.

This fixes the length calculation of the header length.

Fixes: 9213c50e36fa ("app/testpmd: support GTP PSC option in raw sets")
Cc: stable@dpdk.org

Signed-off-by: Raslan Darawsheh <18fd192a829ab99382a9c480ddb6085edf3e8489@nvidia.com>
Reviewed-by: Viacheslav Ovsiienko <5b8db9bb93a9a9aefc8828f79604d1fe963a53dd@nvidia.com>

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