app/testpmd: fix check without outer checksum

app/testpmd: fix check without outer checksum

Donot use outer metadata when neither outer ip checksum nor
outer udp checksum is enabled. PMD's will ignore the
outer_l2_len and outer_l3_len in cases where none of
the outer checksum is enabled and hence only l2_len and
l3_len will be used to calculate the offsets for L2 or L3
header.

Fixes: 3c32113a1aac ("app/testpmd: fix IPv6 tunnel checksum")
Cc: stable@dpdk.org
Signed-off-by: Nithin Dabilpuram <ndabilpuram@marvell.com>
Acked-by: Xiaoyun Li <xiaoyun.li@intel.com>