md/linear: use call_rcu to free obsolete 'conf' structures.

Current, when we update the 'conf' structure, when adding a
drive to a linear array, we keep the old version around until
the array is finally stopped, as it is not safe to free it
immediately.

Now that we have rcu protection on all accesses to 'conf',
we can use call_rcu to free it more promptly.

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 linear_private_data *prev;	/* earlier version */
 	sector_t		array_sectors;
 	dev_info_t		disks[0];
+	struct rcu_head		rcu;
 };