md/linear: shutup lockdep warnning

Commit 03a9e24e(md linear: fix a race between linear_add() and
linear_congested()) introduces the warnning.
Acked-by: Coly Li <colyli@suse.de>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/linear.c

@@ -224,7 +224,8 @@ static int linear_add(struct mddev *mddev, struct md_rdev *rdev)
 	 * oldconf until no one uses it anymore.
 	 */
 	mddev_suspend(mddev);
-	oldconf = rcu_dereference(mddev->private);
+	oldconf = rcu_dereference_protected(mddev->private,
+			lockdep_is_held(&mddev->reconfig_mutex));
 	mddev->raid_disks++;
 	WARN_ONCE(mddev->raid_disks != newconf->raid_disks,
 		"copied raid_disks doesn't match mddev->raid_disks");