dm raid: support raid0 with missing metadata devices

The raid0 MD personality does not start a raid0 array with any of its
data devices missing.

dm-raid was removing data/metadata device pairs unconditionally if it
failed to read a superblock off the respective metadata device of such
pair, resulting in failure to start arrays with the raid0 personality.

Avoid removing any data/metadata device pairs in case of raid0
(e.g. lvm2 segment type 'raid0_meta') thus allowing MD to start the
array.

Also, avoid region size validation for raid0.

Signed-off-by: Heinz Mauelshagen <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-raid.c

···
 {
 	unsigned long min_region_size = rs->ti->len / (1 << 21);
 
+	if (rs_is_raid0(rs))
+		return 0;
+
 	if (!region_size) {
 		/*
 		 * Choose a reasonable default.	 All figures in sectors.
···
 			rebuild_cnt++;
 
 	switch (rs->raid_type->level) {
+	case 0:
+		break;
 	case 1:
 		if (rebuild_cnt >= rs->md.raid_disks)
 			goto too_many;
···
 		case 0:
 			break;
 		default:
+			/*
+			 * We have to keep any raid0 data/metadata device pairs or
+			 * the MD raid0 personality will fail to start the array.
+			 */
+			if (rs_is_raid0(rs))
+				continue;
+
 			dev = container_of(rdev, struct raid_dev, rdev);
 			if (dev->meta_dev)
 				dm_put_device(ti, dev->meta_dev);