md: allow re-add to failed arrays.

When an array is failed (some data inaccessible) then there is no
point attempting to add a spare as it could not possibly be recovered.

However that may be value in re-adding a recently removed device.
e.g. if there is a write-intent-bitmap and it is clear, then access
to the data could be restored by this action.

So don't reject a re-add to a failed array for RAID10 and RAID5 (the
only arrays  types that check for a failed array).
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid10.c

@@ -1483,7 +1483,7 @@ static int raid10_add_disk(struct mddev *mddev, struct md_rdev *rdev)
 		 * very different from resync
 		 */
 		return -EBUSY;
-	if (!enough(conf, -1))
+	if (rdev->saved_raid_disk < 0 && !enough(conf, -1))
 		return -EINVAL;
 
 	if (rdev->raid_disk >= 0)

drivers/md/raid5.c

@@ -5361,7 +5361,7 @@ static int raid5_add_disk(struct mddev *mddev, struct md_rdev *rdev)
 	if (mddev->recovery_disabled == conf->recovery_disabled)
 		return -EBUSY;
 
-	if (has_failed(conf))
+	if (rdev->saved_raid_disk < 0 && has_failed(conf))
 		/* no point adding a device */
 		return -EINVAL;