tty: tty_io: fix driver refcount imbalance on error path

tty_lookup_driver take a reference to the struct tty_driver, but forget
to release it on the error path, lead to a memory leak.
add a tty_driver_kref_put before error return.
Signed-off-by: Lin Yi <teroincn@163.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -1837,7 +1837,7 @@ static struct tty_struct *tty_open_current_tty(dev_t device, struct file *filp)
 static struct tty_driver *tty_lookup_driver(dev_t device, struct file *filp,
 		int *index)
 {
-	struct tty_driver *driver;
+	struct tty_driver *driver = NULL;
 
 	switch (device) {
 #ifdef CONFIG_VT
@@ -1858,6 +1858,8 @@ static struct tty_driver *tty_lookup_driver(dev_t device, struct file *filp,
 				break;
 			}
 		}
+		if (driver)
+			tty_driver_kref_put(driver);
 		return ERR_PTR(-ENODEV);
 	}
 	default: