md: add a new helper rdev_blocked()

The helper will be used in later patches for raid1/raid10/raid5, the
difference is that Faulty rdev with unacknowledged bad block will not
be considered blocked.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Tested-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Link: https://lore.kernel.org/r/20241031033114.3845582-2-yukuai1@huaweicloud.com


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