libbpf: make uniform use of btf__fd() accessor inside libbpf

It makes future grepping and code analysis a bit easier.

Acked-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240104013847.3875810-2-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>