Currently md-cluster will set bitmap->counts.pages directly, add a
helper to do this to avoid dereferencing bitmap directly.

Noted that after this patch bitmap is not dereferenced directly anymore
and following patches will move the structure inside md-bitmap.c.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240826074452.1490072-11-yukuai1@huaweicloud.comSigned-off-by: Song Liu <song@kernel.org>