md: get rdev->mddev with READ_ONCE()

Users may get rdev->mddev by sysfs while rdev is releasing.
So use both READ_ONCE() and WRITE_ONCE() to prevent load/store tearing
and to read/write mddev atomically.

Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231229070500.3602712-1-lilingfeng@huaweicloud.com