md: centralize ->array_sectors modifications

Get personalities out of the business of directly modifying
->array_sectors.  Lays groundwork to introduce policy on when
->array_sectors can be modified.

Reviewed-by: Andre Noll <maan@systemlinux.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 5bf1b24..ad153b2 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -2193,7 +2193,7 @@
 	/*
 	 * Ok, everything is just fine now
 	 */
-	mddev->array_sectors = raid10_size(mddev, 0, 0);
+	md_set_array_sectors(mddev, raid10_size(mddev, 0, 0));
 	mddev->resync_max_sectors = mddev->array_sectors;
 
 	mddev->queue->unplug_fn = raid10_unplug;