In upstream, bitmap file is removed

commit 581ba1341017943d740ed8163d68f04671c2fdde
Author: Yu Kuai <yukuai3@huawei.com>
Date: Mon Dec 2 09:59:13 2024 +0800

mdadm: remove bitmap file support

Because it's marked deprecated for a long time now, and it's not worthy
to support it for new bitmap.

Now that we don't need to store filename for bitmap, also declare a new
enum type bitmap_type to simplify code.