virtio-console: call scheduler when we free unused buffs

For virtio-net we were getting CPU stall warnings, and fixed it by
calling the scheduler: see f8bb51043945 ("virtio_net: suppress cpu stall
when free_unused_bufs").

This driver is similar so theoretically the same logic applies.

Signed-off-by: Xianting Tian <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/char/virtio_console.c

···
 		flush_bufs(vq, true);
 		while ((buf = virtqueue_detach_unused_buf(vq)))
 			free_buf(buf, true);
+		cond_resched();
 	}
 	portdev->vdev->config->del_vqs(portdev->vdev);
 	kfree(portdev->in_vqs);