bpf: Always raise reference in btf_get_module_btf

Align it with helpers like bpf_find_btf_id, so all functions returning
BTF in out parameter follow the same rule of raising reference
consistently, regardless of module or vmlinux BTF.

Adjust existing callers to handle the change accordinly.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220317115957.3193097-10-memxor@gmail.com