samples: bpf: make the use of xdp samples consistent

Currently, each xdp samples are inconsistent in the use.
Most of the samples fetch the interface with it's name.
(ex. xdp1, xdp2skb, xdp_redirect_cpu, xdp_sample_pkts, etc.)

But some of the xdp samples are fetching the interface with
ifindex by command argument.

This commit enables xdp samples to fetch interface with it's name
without changing the original index interface fetching.
(<ifname|ifindex> fetching in the same way as xdp_sample_pkts_user.c does.)

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>