Enable testpmd to forward GTP packet in csum fwd mode.
A GTP header structure (without optional fields and extension header)
is defined in new rte_gtp.h.
A parser function in testpmd is added. GTPU and GTPC packets are both
supported, with respective UDP destination port and GTP message type.
Signed-off-by: Ting Xu <ting.xu@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>