src: udp_flood minimum UDP payload should be 18 bytes.

For the minimum Ethernet size choose UDP payload 18 bytes:
 14(eth) + 8(UDP) + 20(IP) +4(Eth-CRC) - 64 bytes = 18 bytes

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
trafgen: example of UDP flooding with trafgen

I'm using this to overload the udp_sink program.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>