pktgen: add sample script pktgen_sample05_flow_per_thread.sh

This pktgen sample script is useful for scalability testing a
receiver.  The script will simply generate one flow per
thread (option -t N) using the thread number as part of the
source IP-address.

The single flow sample (pktgen_sample03_burst_single_flow.sh)
have become quite popular, but it is important that developers
also make sure to benchmark scalability of multiple receive
queues.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>