To avoid dereferencing bitmap directly in md-cluster to prepare
inventing a new bitmap.

BTW, also fix following checkpatch warnings:

WARNING: Deprecated use of 'kmap_atomic', prefer 'kmap_local_page' instead
WARNING: Deprecated use of 'kunmap_atomic', prefer 'kunmap_local' instead
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240826074452.1490072-7-yukuai1@huaweicloud.comSigned-off-by: Song Liu <song@kernel.org>