USB: fix USB boot crash, ecm_do_notify(), list_add corruption. prev->next should be next (ffff88003b8f82f8)

This fixes a BUG() turned up by Ingo via randconfig testing, where
CONFIG_LIST_DEBUG turned up list corruption.  The corruption was
caused by the dummy_hcd (single-machine test harness for gadget and
HCD code) trashing the request queue when driven by the new CDC
composite gadget an I/O pattern that was previously uncommon.
Fix suggested by Alan Stern.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Tested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/gadget/dummy_hcd.c

@@ -542,13 +542,14 @@ dummy_queue (struct usb_ep *_ep, struct usb_request *_req,
 		req->req.context = dum;
 		req->req.complete = fifo_complete;
 
+		list_add_tail(&req->queue, &ep->queue);
 		spin_unlock (&dum->lock);
 		_req->actual = _req->length;
 		_req->status = 0;
 		_req->complete (_ep, _req);
 		spin_lock (&dum->lock);
-	}
-	list_add_tail (&req->queue, &ep->queue);
+	}  else
+		list_add_tail(&req->queue, &ep->queue);
 	spin_unlock_irqrestore (&dum->lock, flags);
 
 	/* real hardware would likely enable transfers here, in case