md/raid1: use md_bitmap_wait_behind_writes() in raid1_read_request()

Use the existed helper instead of open coding it to make the code cleaner.
There are no functional changes, and also avoid dereferencing bitmap
directly to prepare inventing a new bitmap.

Noted that this patch also export md_bitmap_wait_behind_writes(), which
is necessary for now, and the exported api will be removed in following
patches to convert bitmap apis into ops.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240826074452.1490072-2-yukuai1@huaweicloud.com


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