usbip: fix possibility of dereference by NULLL pointer in vhci_hcd.c

This patch fixes possibility of dereference by NULLL pointer in "[PATCH
v5 1/3] usbip: vhci extension: modifications to vhci driver" which has
been merged to 4.9-rc1. It occurs when a URB with pointer to invalid
USB/IP device is enqueued in race condition against detach operation.

A pointer was passed to vdev_to_vhci() before NULL check.
In vdev_to_vhci(), there's a dereference by the pointer.

This patch moves vdev_to_vhci() after NULL check of the pointer.
Signed-off-by: Nobuo Iwata <nobuo.iwata@fujixerox.co.jp>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/usbip/vhci_hcd.c

@@ -460,13 +460,14 @@ static void vhci_tx_urb(struct urb *urb)
 {
 	struct vhci_device *vdev = get_vdev(urb->dev);
 	struct vhci_priv *priv;
-	struct vhci_hcd *vhci = vdev_to_vhci(vdev);
+	struct vhci_hcd *vhci;
 	unsigned long flags;
 
 	if (!vdev) {
 		pr_err("could not get virtual device");
 		return;
 	}
+	vhci = vdev_to_vhci(vdev);
 
 	priv = kzalloc(sizeof(struct vhci_priv), GFP_ATOMIC);
 	if (!priv) {