wasi: fix poll_oneoff memory interface

The WASM memory interfacing logic was wrong (particularly around
the size of __wasi_subscription_t). This commit fixes the logic.

PR-URL: https://github.com/nodejs/node/pull/33250


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>