vhost: error handling fix

vhost should set worker to NULL on cgroups attach failure,
so that we won't try to destroy the worker again on close.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 1afa085..c579dcc 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -298,6 +298,7 @@
 	return 0;
 err_cgroup:
 	kthread_stop(worker);
+	dev->worker = NULL;
 err_worker:
 	if (dev->mm)
 		mmput(dev->mm);