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>

drivers/md/md.c

@@ -4186,6 +4186,7 @@ size_store(struct mddev *mddev, const char *buf, size_t len)
 		return err;
 	if (mddev->pers) {
 		err = update_size(mddev, sectors);
+		if (err == 0)
 			md_update_sb(mddev, 1);
 	} else {
 		if (mddev->dev_sectors == 0 ||