tools: bpftool: use correct argument in cgroup errors

cgroup code tries to use argv[0] as the cgroup path,
but if it fails uses argv[1] to report errors.

Fixes: 5ccda64d ("bpftool: implement cgroup bpf operations")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/cgroup.c

@@ -168,7 +168,7 @@ static int do_show(int argc, char **argv)
 
 	cgroup_fd = open(argv[0], O_RDONLY);
 	if (cgroup_fd < 0) {
-		p_err("can't open cgroup %s", argv[1]);
+		p_err("can't open cgroup %s", argv[0]);
 		goto exit;
 	}
 
@@ -356,7 +356,7 @@ static int do_attach(int argc, char **argv)
 
 	cgroup_fd = open(argv[0], O_RDONLY);
 	if (cgroup_fd < 0) {
-		p_err("can't open cgroup %s", argv[1]);
+		p_err("can't open cgroup %s", argv[0]);
 		goto exit;
 	}
 
@@ -414,7 +414,7 @@ static int do_detach(int argc, char **argv)
 
 	cgroup_fd = open(argv[0], O_RDONLY);
 	if (cgroup_fd < 0) {
-		p_err("can't open cgroup %s", argv[1]);
+		p_err("can't open cgroup %s", argv[0]);
 		goto exit;
 	}