drm/gem: WARN about unbalanced handle refcounts

Trying to drop a reference we don't have is a pretty serious bug.
Trying to paper over it is an even worse offense.

So scream into dmesg with a big WARN in case that ever happens.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_gem.c

@@ -238,7 +238,7 @@ static void drm_gem_object_handle_free(struct drm_gem_object *obj)
 void
 drm_gem_object_handle_unreference_unlocked(struct drm_gem_object *obj)
 {
-	if (atomic_read(&obj->handle_count) == 0)
+	if (WARN_ON(atomic_read(&obj->handle_count) == 0))
 		return;
 
 	/*