sky2: Lock transmit queue while disabling device

netif_device_detach() does not take the tx_lock, so it's possible that
a call to sky2_xmit_frame is still in progress after
netif_device_detach() is complete.

Take netif_tx_lock() to make sure all transmits have stopped while
we're disabling the devices and that no other CPU is still
transmitting a frame after we've disabling the device.

Proposed fix for "sky2 panic under load" reported by Berck E. Nash.
Signed-off-by: NMike McCormack <mikem@ring3k.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/sky2.c

@@ -3176,7 +3176,9 @@ static void sky2_reset(struct sky2_hw *hw)
 static void sky2_detach(struct net_device *dev)
 {
 	if (netif_running(dev)) {
+		netif_tx_lock(dev);
 		netif_device_detach(dev);	/* stop txq */
+		netif_tx_unlock(dev);
 		sky2_down(dev);
 	}
 }