md: revert change to raid_disks on failure.

If we try to update_raid_disks and it fails, we should put
'delta_disks' back to zero.  This is important because some code,
such as slot_store, assumes that delta_disks has been validated.
Signed-off-by: NNeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -5578,6 +5578,8 @@ static int update_raid_disks(mddev_t *mddev, int raid_disks)
 	mddev->delta_disks = raid_disks - mddev->raid_disks;
 
 	rv = mddev->pers->check_reshape(mddev);
+	if (rv < 0)
+		mddev->delta_disks = 0;
 	return rv;
 }