libbpf: Return -ENODATA for missing btf section

As discussed before, return -ENODATA (No data available) would be more
meaningful than ENOENT (No such file or directory).

Suggested-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221231151436.6541-1-changbin.du@gmail.com