raid5: use flex_array for scribble data

Use flex_array for scribble data. Next patch will batch several stripes
together, so scribble data should be able to cover several stripes, so this
patch also allocates scribble data for stripes across a chunk.
Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.h

@@ -458,15 +458,11 @@ struct r5conf {
 	/* per cpu variables */
 	struct raid5_percpu {
 		struct page	*spare_page; /* Used when checking P/Q in raid6 */
-		void		*scribble;   /* space for constructing buffer
+		struct flex_array *scribble;   /* space for constructing buffer
 					      * lists and performing address
 					      * conversions
 					      */
 	} __percpu *percpu;
-	size_t			scribble_len; /* size of scribble region must be
-					       * associated with conf to handle
-					       * cpu hotplug while reshaping
-					       */
 #ifdef CONFIG_HOTPLUG_CPU
 	struct notifier_block	cpu_notify;
 #endif