app/testpmd: fix L4 length for UDP checksum

app/testpmd: fix L4 length for UDP checksum

testpmd only sets the L4 len in case of TCP packets.
some PMD's like tap rely on mbuf meta data to calculate csum

This will set the L4 len for UDP packets same as TCP

Fixes: 160c3dc9458c ("app/testpmd: introduce IP parsing functions in csum fwd engine")
CC: stable@dpdk.org
Signed-off-by: Raslan Darawsheh <rasland@mellanox.com>
Signed-off-by: Ophir Munk <ophirmu@mellanox.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>