ax25: Fix reference count leak issue of net_device

There is a reference count leak issue of the object "net_device" in
ax25_dev_device_down(). When the ax25 device is shutting down, the
ax25_dev_device_down() drops the reference count of net_device one
or zero times depending on if we goto unlock_put or not, which will
cause memory leak.

In order to solve the above issue, decrease the reference count of
net_device after dev->ax25_ptr is set to null.

Fixes: d01ffb9 ("ax25: add refcount in ax25_dev to avoid UAF bugs")
Suggested-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/7ce3b23a40d9084657ba1125432f0ecc380cbc80.1715247018.git.duoming@zju.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ax25/ax25_dev.c

@@ -118,15 +118,10 @@ void ax25_dev_device_down(struct net_device *dev)
 	list_for_each_entry(s, &ax25_dev_list, list) {
 		if (s == ax25_dev) {
 			list_del(&s->list);
-			goto unlock_put;
+			break;
 		}
 	}
-	dev->ax25_ptr = NULL;
-	spin_unlock_bh(&ax25_dev_lock);
-	ax25_dev_put(ax25_dev);
-	return;
 
-unlock_put:
 	dev->ax25_ptr = NULL;
 	spin_unlock_bh(&ax25_dev_lock);
 	netdev_put(dev, &ax25_dev->dev_tracker);