md: cleanup mddev_create/destroy_serial_pool()

Now that except for stopping the array, all the callers already suspend
the array, there is no need to suspend anymore, hence remove the second
parameter.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231010151958.145896-15-yukuai1@huaweicloud.com