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: 36e04a2d78 ("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>

samples/bpf/xdp2skb_meta.sh

@@ -16,8 +16,8 @@
 BPF_FILE=xdp2skb_meta_kern.o
 DIR=$(dirname $0)
 
-export TC=/usr/sbin/tc
-export IP=/usr/sbin/ip
+[ -z "$TC" ] && TC=tc
+[ -z "$IP" ] && IP=ip
 
 function usage() {
     echo ""
@@ -53,7 +53,7 @@ function _call_cmd() {
     local allow_fail="$2"
     shift 2
     if [[ -n "$VERBOSE" ]]; then
-	echo "$(basename $cmd) $@"
+	echo "$cmd $@"
     fi
     if [[ -n "$DRYRUN" ]]; then
 	return