IO worker attributes should allow expressions

/subsystem=io/worker=default:read-resource-description
{
    ...
        "attributes" => {
            "io-threads" => {
                ...
                "expressions-allowed" => false,
                ...
            },
            "stack-size" => {
                ...
                "expressions-allowed" => false,
                ...
            },
            "task-keepalive" => {
                ...
                "expressions-allowed" => false,
                ...
            },
            "task-max-threads" => {
                ...
                "expressions-allowed" => false,
                ...
            }