net_dma: acquire/release dma channels on ifup/ifdown

The recent dmaengine rework removed the capability to remove dma device
driver modules while net_dma is active.  Rather than notify
dmaengine-clients that channels are trying to be removed, we now rely on
clients to notify dmaengine when they no longer have a need for
channels.  Teach net_dma to release channels by taking dmaengine
references at netdevice open and dropping references at netdevice close.

Acked-by: Maciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -1087,6 +1087,11 @@ int dev_open(struct net_device *dev)
 		 */
 		dev->flags |= IFF_UP;
 
+		/*
+		 *	Enable NET_DMA
+		 */
+		dmaengine_get();
+
 		/*
 		 *	Initialize multicasting status
 		 */
@@ -1164,6 +1169,11 @@ int dev_close(struct net_device *dev)
 	 */
 	call_netdevice_notifiers(NETDEV_DOWN, dev);
 
+	/*
+	 *	Shutdown NET_DMA
+	 */
+	dmaengine_put();
+
 	return 0;
 }
 
@@ -5151,9 +5161,6 @@ static int __init net_dev_init(void)
 	hotcpu_notifier(dev_cpu_callback, 0);
 	dst_init();
 	dev_mcast_init();
-	#ifdef CONFIG_NET_DMA
-	dmaengine_get();
-	#endif
 	rc = 0;
 out:
 	return rc;