dm raid: fix inaccessible superblocks causing oops in configure_discard_support

Commit 48cf06bc ("dm raid: add discard support for RAID levels 4, 5
and 6") did not properly handle missing metadata device(s).  A failing
read of the superblock causes the metadata and data devices to be
removed from the dev array in struct raid_set, setting references to
both devices to NULL.  configure_discard_support() nonetheless tries to
access the data dev unconditionally causing an oops.
Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-raid.c

@@ -1172,8 +1172,12 @@ static void configure_discard_support(struct dm_target *ti, struct raid_set *rs)
 	raid456 = (rs->md.level == 4 || rs->md.level == 5 || rs->md.level == 6);
 
 	for (i = 0; i < rs->md.raid_disks; i++) {
-		struct request_queue *q = bdev_get_queue(rs->dev[i].rdev.bdev);
+		struct request_queue *q;
 
+		if (!rs->dev[i].rdev.bdev)
+			continue;
+
+		q = bdev_get_queue(rs->dev[i].rdev.bdev);
 		if (!q || !blk_queue_discard(q))
 			return;