md: add helper rdev_needs_recovery()

Add a helper for checking if an rdev needs recovery.

Signed-off-by: Zheng Qixing <zhengqixing@huawei.com>
Link: https://lore.kernel.org/linux-raid/20250816002534.1754356-2-zhengqixing@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>