Unexpected result from example code in `/2022-02-io_uring-worker-pool`

Hello,
thanks for your example code and blog post.

I execute the program `udp-read` with argument `--async --threads 2 --rings 1`
However, I always get `Error: Invalid argument (os error 22)` (even I changed the arguments).

I'm wondering do I missing some details? (I'm not familiar with rust)

Configuration:
rustc version: `1.56.1`
cargo version: `1.56.0`

Start in /2022-02-io_uring-worker-pool and locate the udp-read entry point. Run the reported command with Rust 1.56.1 and Cargo 1.56.0, then compare it with the changed --async, --threads, and --rings arguments; done means identifying why the example returns os error 22 and documenting or correcting the reproducible behavior.

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