drm/i915: Initialise the obj->rcu head

Make the rcu_head known to the system, in particular for debugobjects.
And having declared it for debugobjects, we need to tidy up afterwards.

v2: mark the obj->rcu as being destroyed when we reuse its location for
the freed list.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108691Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181109090311.15321-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -4739,6 +4739,8 @@ void i915_gem_object_init(struct drm_i915_gem_object *obj,
 	INIT_LIST_HEAD(&obj->lut_list);
 	INIT_LIST_HEAD(&obj->batch_pool_link);
 
+	init_rcu_head(&obj->rcu);
+
 	obj->ops = ops;
 
 	reservation_object_init(&obj->__builtin_resv);
@@ -5005,6 +5007,13 @@ static void __i915_gem_free_object_rcu(struct rcu_head *head)
 		container_of(head, typeof(*obj), rcu);
 	struct drm_i915_private *i915 = to_i915(obj->base.dev);
 
+	/*
+	 * We reuse obj->rcu for the freed list, so we had better not treat
+	 * it like a rcu_head from this point forwards. And we expect all
+	 * objects to be freed via this path.
+	 */
+	destroy_rcu_head(&obj->rcu);
+
 	/*
 	 * Since we require blocking on struct_mutex to unbind the freed
 	 * object from the GPU before releasing resources back to the