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>