md/md-bitmap: replace md_bitmap_status() with a new helper md_bitmap_get_stats()

There are no functional changes, and the new helper will be used in
multiple places in following patches to avoid dereferencing bitmap
directly.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240826074452.1490072-3-yukuai1@huaweicloud.com


Signed-off-by: Song Liu <song@kernel.org>