md/raid5: don't include 'spare' drives when reshaping to fewer devices.

There are few situations where it would make any sense to add a spare
when reducing the number of devices in an array, but it is
conceivable:  A 6 drive RAID6 with two missing devices could be
reshaped to a 5 drive RAID6, and a spare could become available
just in time for the reshape, but not early enough to have been
recovered first.  'freezing' recovery can make this easy to
do without any races.

However doing such a thing is a bad idea.  md will not record the
partially-recovered state of the 'spare' and when the reshape
finished it will think that the spare is still spare.
Easiest way to avoid this confusion is to simply disallow it.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -5526,7 +5526,12 @@ static int raid5_start_reshape(mddev_t *mddev)
 
 	/* Add some new drives, as many as will fit.
 	 * We know there are enough to make the newly sized array work.
+	 * Don't add devices if we are reducing the number of
+	 * devices in the array.  This is because it is not possible
+	 * to correctly record the "partially reconstructed" state of
+	 * such devices during the reshape and confusion could result.
 	 */
+	if (mddev->delta_disks >= 0)
 	    list_for_each_entry(rdev, &mddev->disks, same_set)
 		if (rdev->raid_disk < 0 &&
 		    !test_bit(Faulty, &rdev->flags)) {
@@ -5549,7 +5554,7 @@ static int raid5_start_reshape(mddev_t *mddev)
 		}
 
 	/* When a reshape changes the number of devices, ->degraded
-	 * is measured against the large of the pre and post number of
+	 * is measured against the larger of the pre and post number of
 	 * devices.*/
 	if (mddev->delta_disks > 0) {
 		spin_lock_irqsave(&conf->device_lock, flags);