app/testpmd: fix TSO with checksum engine

app/testpmd: fix TSO with checksum engine

The commit that disabled TSO for small packets was broken during the
rebase. The problem is the IP checksum is not calculated in software if:
- TX IP checksum is disabled
- TSO is enabled
- the current packet is smaller than tso segment size

When checking if the PKT_TX_IP_CKSUM flag should be set (in case
of tso), use the local tso_segsz variable, which is set to 0 when the
packet is too small to require tso. Therefore the IP checksum will be
correctly calculated in software.

Moreover, we should not use tunnel segment size for non-tunnel tso, else
TSO will stay disabled for all packets.

Fixes: 97c21329d42b ("app/testpmd: do not use TSO for small packets")
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>