app/testpmd: fix txonly flow generation entropy

app/testpmd: fix txonly flow generation entropy

The testpmd application in txonly forwarding mode has an option
to generate the packet flows by varying the destination IP address.
The patch increments the IP for each packet sent, this improves
the entropy and RSS distribution on the peer receiving size
is getting more uniform.

Fixes: 01b645dcff7f ("app/testpmd: move txonly prepare in separate function")
Cc: stable@dpdk.org
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>