samples: pktgen: allow to specify destination port

Currently, kernel pktgen has the feature to specify udp destination port
for sending packet. (e.g. pgset "udp_dst_min 9")

But on samples, each of the scripts doesn't have any option to achieve this.

This commit adds the DST_PORT option to specify the target port(s) in the script.

    -p : ($DST_PORT)  destination PORT range (e.g. 433-444) is also allowed

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>