samples: pktgen: add missing IPv6 option to pktgen scripts

Currently, "sample04" and "sample05" are not working properly when
running with an IPv6 option("-6"). The commit 0f06a678 ("samples:
Add an IPv6 "-6" option to the pktgen scripts") has omitted the addition
of this option at "sample04" and "sample05".

In order to support IPv6 option, this commit adds logic related to IPv6
option.

Fixes: 0f06a678 ("samples: Add an IPv6 "-6" option to the pktgen scripts")

Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>