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 <[e-mail]>

drivers/vhost/vhost.c

···
 	return 0;
 err_cgroup:
 	kthread_stop(worker);
+	dev->worker = NULL;
 err_worker:
 	if (dev->mm)
 		mmput(dev->mm);