drm/gem: Check locking in drm_gem_object_unreference

Pretty soon only some drivers will need dev->struct_mutex in their
gem_free_object callbacks. Hence it's really important to make sure
everything still keeps getting this right.

v2: Don't check for locking before we check for non-NULL obj. Spotted
by Dan Carpenter.

Link: http://mid.gmane.org/1444894601-5200-10-git-send-email-daniel.vetter@ffwll.ch
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

include/drm/drm_gem.h

@@ -142,8 +142,11 @@ drm_gem_object_reference(struct drm_gem_object *obj)
 static inline void
 drm_gem_object_unreference(struct drm_gem_object *obj)
 {
-	if (obj != NULL)
+	if (obj != NULL) {
+		WARN_ON(!mutex_is_locked(&obj->dev->struct_mutex));
+
 		kref_put(&obj->refcount, drm_gem_object_free);
+	}
 }
 
 static inline void