drm/i915: Track rcu_head for our idle worker

While our little rcu worker might be able to be replaced now by the
dedicated rcu_work, in the meantime we should mark up the rcu_head for
correct debugobjects tracking.
Signed-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/20181108092101.27598-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -3557,6 +3557,8 @@ static void __sleep_rcu(struct rcu_head *rcu)
 	struct sleep_rcu_work *s = container_of(rcu, typeof(*s), rcu);
 	struct drm_i915_private *i915 = s->i915;
 
+	destroy_rcu_head(&s->rcu);
+
 	if (same_epoch(i915, s->epoch)) {
 		INIT_WORK(&s->work, __sleep_work);
 		queue_work(i915->wq, &s->work);
@@ -3673,6 +3675,7 @@ i915_gem_idle_work_handler(struct work_struct *work)
 	if (same_epoch(dev_priv, epoch)) {
 		struct sleep_rcu_work *s = kmalloc(sizeof(*s), GFP_KERNEL);
 		if (s) {
+			init_rcu_head(&s->rcu);
 			s->i915 = dev_priv;
 			s->epoch = epoch;
 			call_rcu(&s->rcu, __sleep_rcu);