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/multipath.c b/drivers/md/multipath.c
index a3f6d86..41ced0c 100644
--- a/drivers/md/multipath.c
+++ b/drivers/md/multipath.c
@@ -510,7 +510,7 @@
 	/*
 	 * Ok, everything is just fine now
 	 */
-	mddev->array_sectors = multipath_size(mddev, 0, 0);
+	md_set_array_sectors(mddev, multipath_size(mddev, 0, 0));
 
 	mddev->queue->unplug_fn = multipath_unplug;
 	mddev->queue->backing_dev_info.congested_fn = multipath_congested;