MD:Update superblock when err == 0 in size_store

This is a simple check before updating the superblock. It should update
the superblock when update_size return 0.

Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Shaohua Li <shli@fb.com>
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 43728a4..d0c1e79 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -4186,7 +4186,8 @@
 		return err;
 	if (mddev->pers) {
 		err = update_size(mddev, sectors);
-		md_update_sb(mddev, 1);
+		if (err == 0)
+			md_update_sb(mddev, 1);
 	} else {
 		if (mddev->dev_sectors == 0 ||
 		    mddev->dev_sectors > sectors)