md/raid5,6: add percpu scribble region for buffer lists

Use percpu memory rather than stack for storing the buffer lists used in
parity calculations.  Include space for dma address conversions and pass
that to async_tx via the async_submit_ctl.scribble pointer.

[ Impact: move memory pressure from stack to heap ]

Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/md/raid5.h

@@ -386,7 +386,15 @@ struct raid5_private_data {
 	/* per cpu variables */
 	struct raid5_percpu {
 		struct page	*spare_page; /* Used when checking P/Q in raid6 */
+		void		*scribble;   /* space for constructing buffer
+					      * lists and performing address
+					      * conversions
+					      */
 	} *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