[media] media-entity: cache media_device on object removal

As pointed by Dan, the commit f8fd4c61b5ae ("[media] media-entity:
protect object creation/removal using spin lock")' leads to the
following static checker warning:

	drivers/media/media-entity.c:781 media_remove_intf_link()
	error: dereferencing freed memory 'link'

drivers/media/media-entity.c
   777  void media_remove_intf_link(struct media_link *link)
   778  {
   779          spin_lock(&link->graph_obj.mdev->lock);
   780          __media_remove_intf_link(link);
   781          spin_unlock(&link->graph_obj.mdev->lock);

In practice, I didn't see any troubles even with KASAN enabled. I guess
gcc optimizer internally cached the mdev reference, instead of getting
it twice. Yet, it is a very bad idea to rely on such optimization. So,
let's fix the code.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/media-entity.c

@@ -580,13 +580,15 @@ EXPORT_SYMBOL_GPL(__media_entity_remove_links);
 
 void media_entity_remove_links(struct media_entity *entity)
 {
+	struct media_device *mdev = entity->graph_obj.mdev;
+
 	/* Do nothing if the entity is not registered. */
-	if (entity->graph_obj.mdev == NULL)
+	if (mdev == NULL)
 		return;
 
-	spin_lock(&entity->graph_obj.mdev->lock);
+	spin_lock(&mdev->lock);
 	__media_entity_remove_links(entity);
-	spin_unlock(&entity->graph_obj.mdev->lock);
+	spin_unlock(&mdev->lock);
 }
 EXPORT_SYMBOL_GPL(media_entity_remove_links);
 
@@ -781,9 +783,15 @@ EXPORT_SYMBOL_GPL(__media_remove_intf_link);
 
 void media_remove_intf_link(struct media_link *link)
 {
-	spin_lock(&link->graph_obj.mdev->lock);
+	struct media_device *mdev = link->graph_obj.mdev;
+
+	/* Do nothing if the intf is not registered. */
+	if (mdev == NULL)
+		return;
+
+	spin_lock(&mdev->lock);
 	__media_remove_intf_link(link);
-	spin_unlock(&link->graph_obj.mdev->lock);
+	spin_unlock(&mdev->lock);
 }
 EXPORT_SYMBOL_GPL(media_remove_intf_link);
 
@@ -799,12 +807,14 @@ EXPORT_SYMBOL_GPL(__media_remove_intf_links);
 
 void media_remove_intf_links(struct media_interface *intf)
 {
+	struct media_device *mdev = intf->graph_obj.mdev;
+
 	/* Do nothing if the intf is not registered. */
-	if (intf->graph_obj.mdev == NULL)
+	if (mdev == NULL)
 		return;
 
-	spin_lock(&intf->graph_obj.mdev->lock);
+	spin_lock(&mdev->lock);
 	__media_remove_intf_links(intf);
-	spin_unlock(&intf->graph_obj.mdev->lock);
+	spin_unlock(&mdev->lock);
 }
 EXPORT_SYMBOL_GPL(media_remove_intf_links);