examples/multi_process: fix Rx packets distribution

examples/multi_process: fix Rx packets distribution

MP servers distributes Rx packets between clients according to
round-robin scheme.

Current implementation always started packets distribution from
the first client. That procedure resulted in uniform distribution
in cases when Rx packets number was around clients number
multiplication. However, if RX burst repeatedly returned single
packet, round-robin scheme would not work because all packets
were assigned to the first client only.

The patch does not restart packets distribution from
the first client.
Packets distribution always continues to the next client.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>