pktgen: display IPv4 address in human-readable format

It is weird to display IPv4 address in %x format, what's more,
IPv6 address is disaplayed in human-readable format too. So,
make it human-readable.

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>