Staging: usbip: vhci-hcd: Do not kill already dead RX/TX kthread

When unbinding a device on the host which was still attached on the
client, I got a NULL pointer dereference on the client. This turned out
to be due to kthread_stop() being called on an already dead kthread.

Here is how I was able to reproduce the problem:

 server:# usbip bind -b 1-2
                                client:# usbip attach -h server -b 1-2
 server:# usbip unbind -b 1-2

This patch fixes the problem by checking the kthread before attempting
to kill it, as it is done on the opposite side in
stub_shutdown_connection().
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/usbip/vhci_hcd.c

@@ -860,9 +860,9 @@ static void vhci_shutdown_connection(struct usbip_device *ud)
 	}
 
 	/* kill threads related to this sdev, if v.c. exists */
-	if (vdev->ud.tcp_rx)
+	if (vdev->ud.tcp_rx && !task_is_dead(vdev->ud.tcp_rx))
 		kthread_stop(vdev->ud.tcp_rx);
-	if (vdev->ud.tcp_tx)
+	if (vdev->ud.tcp_tx && !task_is_dead(vdev->ud.tcp_tx))
 		kthread_stop(vdev->ud.tcp_tx);
 
 	pr_info("stop threads\n");