usbip: vudc: fix null pointer dereference on udc->lock

Currently the driver attempts to spin lock on udc->lock before a NULL
pointer check is performed on udc, hence there is a potential null
pointer dereference on udc->lock.  Fix this by moving the null check
on udc before the lock occurs.

Fixes: ea6873a45a22 ("usbip: vudc: Add SysFS infrastructure for VUDC")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Reviewed-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/usbip/vudc_sysfs.c

@@ -105,10 +105,14 @@ static ssize_t usbip_sockfd_store(struct device *dev, struct device_attribute *a
 	if (rv != 0)
 		return -EINVAL;
 
+	if (!udc) {
+		dev_err(dev, "no device");
+		return -ENODEV;
+	}
 	spin_lock_irqsave(&udc->lock, flags);
 	/* Don't export what we don't have */
-	if (!udc || !udc->driver || !udc->pullup) {
-		dev_err(dev, "no device or gadget not bound");
+	if (!udc->driver || !udc->pullup) {
+		dev_err(dev, "gadget not bound");
 		ret = -ENODEV;
 		goto unlock;
 	}