md: Don't flush sync_work in md_write_start()

Because flush sync_work may trigger mddev_suspend() if there are spares,
and this should never be done in IO path because mddev_suspend() is used
to wait for IO.

This problem is found by code review.

Fixes: bc08041b ("md: suspend array in md_start_sync() if array need reconfiguration")
Cc: stable@vger.kernel.org
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240801124746.242558-1-yukuai1@huaweicloud.com


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