samples: bpf: user proper argument index

Use optind as index for argv instead of a hardcoded value.
When the program has options this leads to improper parameter handling.

Fixes: dc378a1a ("samples: bpf: get ifindex from ifname")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Acked-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>