selftests/bpf: Use bpf_link__destroy in fill_link_info tests

The fill_link_info test keeps skeleton open and just creates
various links. We are wrongly calling bpf_link__detach after
each test to close them, we need to call bpf_link__destroy.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/bpf/20231125193130.834322-5-jolsa@kernel.org