libbpf: Relax check whether BTF is mandatory

If BPF program is using BTF-defined maps, BTF is required only for
libbpf itself to process map definitions. If after that BTF fails to
be loaded into kernel (e.g., if it doesn't support BTF at all), this
shouldn't prevent valid BPF program from loading. Existing
retry-without-BTF logic for creating maps will succeed to create such
maps without any problems. So, presence of .maps section shouldn't make
BTF required for kernel. Update the check accordingly.

Validated by ensuring simple BPF program with BTF-defined maps is still
loaded on old kernel without BTF support and map is correctly parsed and
created.

Fixes: abd29c93

 ("libbpf: allow specifying map definitions using BTF")
Reported-by: Julia Kartseva <hex@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200220062635.1497872-1-andriin@fb.com