samples: pktgen: allow to specify destination IP range (CIDR)

Currently, kernel pktgen has the feature to specify destination
address range for sending packet. (e.g. pgset "dst_min/dst_max")

But on samples, each pktgen script doesn't have any option to achieve this.

This commit adds the feature to specify the destination address range with CIDR.

    -d : ($DEST_IP)   destination IP. CIDR (e.g. 198.18.0.0/15) is also allowed

    # ./pktgen_sample01_simple.sh -6 -d fe80::20/126 -p 3000 -n 4
    # tcpdump ip6 and udp
    05:14:18.082285 IP6 fe80::99.71 > fe80::23.3000: UDP, length 16
    05:14:18.082564 IP6 fe80::99.43 > fe80::23.3000: UDP, length 16
    05:14:18.083366 IP6 fe80::99.107 > fe80::22.3000: UDP, length 16
    05:14:18.083585 IP6 fe80::99.97 > fe80::21.3000: UDP, length 16

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>