xhci: Fix use-after-free in xhci_free_virt_device

KASAN found a use-after-free in xhci_free_virt_device+0x33b/0x38e where xhci_free_virt_device() sets slot id to 0 if udev exists:
if (dev->udev && dev->udev->slot_id)

	dev->udev->slot_id = 0;

dev->udev will be true even if udev is freed because dev->udev is not set to NULL.

set dev->udev pointer to NULL in xhci_free_dev()

The original patch went to stable so this fix needs to be applied there as well.

---
 drivers/usb/host/xhci.c | 1 +
 1 file changed, 1 insertion(+)

index 9b27798..711da33 100644
@@ -3621,6 +3621,7 @@ static void xhci_free_dev(struct usb_hcd *hcd, struct usb_device *udev)
 		del_timer_sync(&virt_dev->eps[i].stop_cmd_timer);
 	}
 	xhci_debugfs_remove_slot(xhci, udev->slot_id);
+	virt_dev->udev = NULL;
 	ret = xhci_disable_slot(xhci, udev->slot_id);
 	if (ret)
 		xhci_free_virt_device(xhci, udev->slot_id);

2.7.4