USB: misc/vstusb, fix lock imbalance

Make sure we don't leak locked vstdev->lock in vstusb_write. Unlock
properly on one fail path.

Signed-off-by: Jiri Slaby <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/misc/vstusb.c

···
 	}
 
 	if (copy_from_user(buf, buffer, count)) {
+		mutex_unlock(&vstdev->lock);
 		dev_err(&dev->dev, "%s: can't copy_from_user\n", __func__);
 		retval = -EFAULT;
 		goto exit;