usb: gadget: u_ether: synchronize with transmit when stopping queue

When disconnecting, it's possible that another thread has already made it
into eth_start_xmit before we call netif_stop_queue. This can lead to a
crash as eth_start_xmit tries to use resources that gether_disconnect is
freeing. Use netif_tx_lock/unlock around netif_stop_queue to ensure no
threads are executing during the remainder of gether_disconnect.

Signed-off-by: Jeff Westfahl <jeff.westfahl@ni.com>
Tested-by: Jaeden Amero <jaeden.amero@ni.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/u_ether.c

@@ -1120,7 +1120,10 @@ void gether_disconnect(struct gether *link)
 
  DBG(dev, "%s\n", __func__);
 
+ netif_tx_lock(dev->net);
  netif_stop_queue(dev->net);
+ netif_tx_unlock(dev->net);
+
  netif_carrier_off(dev->net);
 
  /* disable endpoints, forcing (synchronous) completion