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 <jirislaby@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/usb/misc/vstusb.c b/drivers/usb/misc/vstusb.c
index 63dff9b..f26ea8d 100644
--- a/drivers/usb/misc/vstusb.c
+++ b/drivers/usb/misc/vstusb.c
@@ -401,6 +401,7 @@
 	}
 
 	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;