userdata lead to memory leak in fuse demo

In the fuse [demo](https://github.com/deepseek-ai/3FS/blob/main/hf3fs_fuse/fuse_demo.py#L20)
```
ios = [(iov[:512], fd, 512), (iov[512:], fd, 0)] # iov, fd, offset
for io in ios:
ior.prepare(io[0], True, io[1], io[2], userdata=io) # iov, for_read, fd, offset, userdata
# Only for_read == True is allowed, because ior has for_read == True
# userdata must be a referenced python object, we reference io in the list ios, so it will not be sent to GC
resv = ior.submit().wait(min_results=2)
```
Passing ```io``` as ```userdata``` will prevent ```io``` from being released. May be a misleading of ```userdata``` usage?