md/raid5: prepare for allowing reshape to change layout

Add prev_algo to raid5_conf_t along the same lines as prev_chunk
and previous_raid_disks.

Signed-off-by: NeilBrown <neilb@suse.de>
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index b9c9328..cdd0456 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -350,7 +350,7 @@
 	 */
 	sector_t		reshape_safe;
 	int			previous_raid_disks;
-	int			prev_chunk;
+	int			prev_chunk, prev_algo;
 	short			generation; /* increments with every reshape */
 
 	struct list_head	handle_list; /* stripes needing handling */