libbpf: Simplify BTF initialization logic

Current implementation of bpf_object's BTF initialization is very convoluted
and thus prone to errors. It doesn't have to be like that. This patch
simplifies it significantly.

This code also triggered static analysis issues over logically dead code due
to redundant error checks. This simplification should fix that as well.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200117060801.1311525-3-andriin@fb.com