md/raid6: remove expectation that Q device is immediately after P device.

Code currently assumes that the devices in a raid6 stripe are
  0 1 ... N-1 P Q
in some rotated order.  We will shortly add new layouts in which
this strict pattern is broken.
So remove this expectation.  We still assume that the data disks
are roughly in-order.  However P and Q can be inserted anywhere within
that order.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.h

@@ -199,7 +199,8 @@ struct stripe_head {
 	struct list_head	lru;	      /* inactive_list or handle_list */
 	struct raid5_private_data *raid_conf;
 	sector_t		sector;		/* sector of this row */
-	int			pd_idx;			/* parity disk index */
+	short			pd_idx;		/* parity disk index */
+	short			qd_idx;		/* 'Q' disk index for raid6 */
 	unsigned long		state;		/* state flags */
 	atomic_t		count;	      /* nr of active thread/requests */
 	spinlock_t		lock;