libbpf: Simplify the return expression of bpf_object__init_maps function

There is no need for special treatment of the 'ret == 0' case.
This patch simplifies the return expression.

Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210609115651.3392580-1-wanghai38@huawei.com