md/md-bitmap: merge md_bitmap_unplug_async() into md_bitmap_unplug()

Add a parameter 'bool sync' to distinguish them, and
md_bitmap_unplug_async() won't be exported anymore, hence
bitmap_operations only need one op to cover them.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240826074452.1490072-32-yukuai1@huaweicloud.com


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