samples: pktgen: add helper functions for IP(v4/v6) CIDR parsing

This commit adds CIDR parsing and IP validate helper function to parse
single IP or range of IP with CIDR. (e.g. 198.18.0.0/15)

Validating the address should be preceded prior to the parsing.
Helpers will be used in prior to set target address in samples/pktgen.

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>