md/md-bitmap: merge md_bitmap_wait_behind_writes() into bitmap_operations

So that the implementation won't be exposed, and it'll be possible
to invent a new bitmap by replacing bitmap_operations.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240826074452.1490072-41-yukuai1@huaweicloud.com


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