drm/exynos: fix race condition UAF in exynos_g2d_exec_ioctl

If it is async, runqueue_node is freed in g2d_runqueue_worker on another
worker thread. So in extreme cases, if g2d_runqueue_worker runs first, and
then executes the following if statement, there will be use-after-free.
Signed-off-by: Min Li <lm0963hack@gmail.com>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_g2d.c

@@ -1335,7 +1335,7 @@ int exynos_g2d_exec_ioctl(struct drm_device *drm_dev, void *data,
 	/* Let the runqueue know that there is work to do. */
 	queue_work(g2d->g2d_workq, &g2d->runqueue_work);
 
-	if (runqueue_node->async)
+	if (req->async)
 		goto out;
 
 	wait_for_completion(&runqueue_node->complete);