libbpf: Return err if bpf_object__load failed

bpf_object__load() has various return code, when it failed to load
object, it must return err instead of -EINVAL.

Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200426063635.130680-3-maowenan@huawei.com