md/linear: avoid corrupting structure while waiting for rcu_free to complete.

I don't know what I was thinking putting 'rcu' after a dynamically
sized array!  The array could still be in use when we call rcu_free()
(That is the point) so we mustn't corrupt it.

Cc: stable@kernel.org
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/linear.h

@@ -10,9 +10,9 @@ typedef struct dev_info dev_info_t;
 
 struct linear_private_data
 {
+	struct rcu_head		rcu;
 	sector_t		array_sectors;
 	dev_info_t		disks[0];
-	struct rcu_head		rcu;
 };