libbpf: Change log level of BTF loading error message

Reduce log level of BTF loading error to INFO if BTF is not required.

Andrii says:

  Nowadays the expectation is that the BPF program will have a valid
  .BTF section, so even though .BTF is "optional", I think it's fine
  to emit a warning for that case (any reasonably recent Clang will
  produce valid BTF).

  Ihor's patch is fixing the situation with an outdated host kernel
  that doesn't understand BTF. libbpf will try to "upload" the
  program's BTF, but if that fails and the BPF object doesn't use
  any features that require having BTF uploaded, then it's just an
  information message to the user, but otherwise can be ignored.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Ihor Solodrai <ihor.solodrai@pm.me>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>