samples/bpf: Fix tc and ip paths in xdp2skb_meta.sh

The below path error can occur:

  # ./xdp2skb_meta.sh --dev eth0 --list
  ./xdp2skb_meta.sh: line 61: /usr/sbin/tc: No such file or directory

So just use command names instead of absolute paths of tc and ip.
In addition, it allow callers to redefine $TC and $IP paths

Fixes: 36e04a2d ("samples/bpf: xdp2skb_meta shows transferring info from XDP to SKB")
Reviewed-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>