IB/ipoib: Use cancel_delayed_work_sync when needed

The work mcast_task can re-queue itself, so instead of doing
cancel && flush_workqueue, that still can leave a queued task
on the air, use cancel_delayed_work_sync.

Also, no need to use lock over the cancel, the original lock was
due to bit assignment setting (IPOIB_MCAST_RUN) that is not in use
anymore.
Signed-off-by: NErez Shitrit <erezsh@mellanox.com>
Signed-off-by: NLeon Romanovsky <leon@kernel.org>

drivers/infiniband/ulp/ipoib/ipoib_multicast.c

@@ -684,15 +684,10 @@ void ipoib_mcast_start_thread(struct net_device *dev)
 int ipoib_mcast_stop_thread(struct net_device *dev)
 {
 	struct ipoib_dev_priv *priv = ipoib_priv(dev);
-	unsigned long flags;
 
 	ipoib_dbg_mcast(priv, "stopping multicast thread\n");
 
-	spin_lock_irqsave(&priv->lock, flags);
-	cancel_delayed_work(&priv->mcast_task);
-	spin_unlock_irqrestore(&priv->lock, flags);
-
-	flush_workqueue(priv->wq);
+	cancel_delayed_work_sync(&priv->mcast_task);
 
 	return 0;
 }