watchdog: Ensure that wdd is not dereferenced if NULL

Smatch rightfully complains that wdd is dereferenced in the watchdog
release function after being checked for NULL. Also make sure that it
is not accessed outside mutex protection to avoid use-after-free problems.

Fixes: e6c71e84e4c0 ("watchdog: Introduce WDOG_HW_RUNNING flag")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

drivers/watchdog/watchdog_dev.c

@@ -711,6 +711,7 @@ static int watchdog_release(struct inode *inode, struct file *file)
 	struct watchdog_core_data *wd_data = file->private_data;
 	struct watchdog_device *wdd;
 	int err = -EBUSY;
+	bool running;
 
 	mutex_lock(&wd_data->lock);
 
@@ -742,14 +743,15 @@ static int watchdog_release(struct inode *inode, struct file *file)
 	clear_bit(_WDOG_DEV_OPEN, &wd_data->status);
 
 done:
+	running = wdd && watchdog_hw_running(wdd);
 	mutex_unlock(&wd_data->lock);
 	/*
 	 * Allow the owner module to be unloaded again unless the watchdog
 	 * is still running. If the watchdog is still running, it can not
 	 * be stopped, and its driver must not be unloaded.
 	 */
-	if (!watchdog_hw_running(wdd)) {
-		module_put(wdd->ops->owner);
+	if (!running) {
+		module_put(wd_data->cdev.owner);
 		kref_put(&wd_data->kref, watchdog_core_data_release);
 	}
 	return 0;