md: suspend array in md_start_sync() if array need reconfiguration

So that io won't concurrent with array reconfiguration, and it's safe to
suspend the array directly because normal io won't rely on
md_start_sync().

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231010151958.145896-18-yukuai1@huaweicloud.com