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/faulty.c b/drivers/md/faulty.c
index d76c87f..8695809 100644
--- a/drivers/md/faulty.c
+++ b/drivers/md/faulty.c
@@ -312,7 +312,7 @@
 	list_for_each_entry(rdev, &mddev->disks, same_set)
 		conf->rdev = rdev;
 
-	mddev->array_sectors = faulty_size(mddev, 0, 0);
+	md_set_array_sectors(mddev, faulty_size(mddev, 0, 0));
 	mddev->private = conf;
 
 	reconfig(mddev, mddev->layout, -1);