xen-pcifront: Handle backend CLOSED without CLOSING

Backend drivers shouldn't transition to CLOSED unless the frontend is
CLOSED.  If a backend does transition to CLOSED too soon then the
frontend may not see the CLOSING state and will not properly shutdown.

So, treat an unexpected backend CLOSED state the same as CLOSING.
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/pci/xen-pcifront.c

@@ -1068,13 +1068,16 @@ static void __init_refok pcifront_backend_changed(struct xenbus_device *xdev,
 	case XenbusStateInitialising:
 	case XenbusStateInitWait:
 	case XenbusStateInitialised:
-	case XenbusStateClosed:
 		break;
 
 	case XenbusStateConnected:
 		pcifront_try_connect(pdev);
 		break;
 
+	case XenbusStateClosed:
+		if (xdev->state == XenbusStateClosed)
+			break;
+		/* Missed the backend's CLOSING state -- fallthrough */
 	case XenbusStateClosing:
 		dev_warn(&xdev->dev, "backend going away!\n");
 		pcifront_try_disconnect(pdev);