samples: Add an IPv6 '-6' option to the pktgen scripts

Add a '-6' option to the sample pktgen scripts for sending out
IPv6 packets.

[root@kerneldev010.prn1 ~/pktgen]# ./pktgen_sample03_burst_single_flow.sh -i eth0 -s 64 -d fe80::f652:14ff:fec2:a14c -m f4:52:14:c2:a1:4c -b 32 -6

[root@kerneldev011.prn1 ~]# tcpdump -i eth0 -nn -c3 port 9
tcpdump: WARNING: eth0: no IPv4 address assigned
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
14:38:51.815297 IP6 fe80::f652:14ff:fec2:2ad2.9 > fe80::f652:14ff:fec2:a14c.9: UDP, length 16
14:38:51.815311 IP6 fe80::f652:14ff:fec2:2ad2.9 > fe80::f652:14ff:fec2:a14c.9: UDP, length 16
14:38:51.815313 IP6 fe80::f652:14ff:fec2:2ad2.9 > fe80::f652:14ff:fec2:a14c.9: UDP, length 16

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>