md/raid5: allow 5-device RAID6 to be reshaped to 4-device.

There is a bug in 'check_reshape' for raid5.c  To checks
that the new minimum number of devices is large enough (which is
good), but it does so also after the reshape has started (bad).

This is bad because
 - the calculation is now wrong as mddev->raid_disks has changed
   already, and
 - it is pointless because it is now too late to stop.

So only perform that test when reshape has not been committed to.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -5914,7 +5914,7 @@ static int check_reshape(struct mddev *mddev)
 		return 0; /* nothing to do */
 	if (has_failed(conf))
 		return -EINVAL;
-	if (mddev->delta_disks < 0) {
+	if (mddev->delta_disks < 0 && mddev->reshape_position == MaxSector) {
 		/* We might be able to shrink, but the devices must
 		 * be made bigger first.
 		 * For raid6, 4 is the minimum size.