md: use new helper md_bitmap_get_stats() in update_array_info()

There are no functional changes, avoid dereferencing bitmap directly to
prepare inventing a new bitmap.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240826074452.1490072-4-yukuai1@huaweicloud.com


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