app/testpmd: fix GTP header parsing in checksum engine

app/testpmd: fix GTP header parsing in checksum engine

GTP header can be followed by an optional 32 bits extension.
GTP notifies about the extension presence through the E, S or PN
header bits.

Csum GTP header parser did not check the extension bits value.

The patch updates GTP header length if at-least one of the
extension bits is set.

Fixes: d8e5e69f3a9b ("app/testpmd: add GTP parsing and Tx checksum offload")
Cc: stable@dpdk.org
Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Aman Singh <aman.deep.singh@intel.com>