pktgen: add benchmark script pktgen_bench_xmit_mode_netif_receive.sh

This script pktgen_bench_xmit_mode_netif_receive.sh is a benchmark
script, which can be used for benchmarking part of the network stack.
This can be used for performance improving or catching regression in
that area.

The script is developed for benchmarking ingress qdisc path, original
idea by Alexei Starovoitov.  This script don't really need any
hardware.  This is achieved via the recently introduced stack inject
feature "xmit_mode netif_receive". See commit 62f64aed ("pktgen:
introduce xmit_mode '<start_xmit|netif_receive>'").

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>