aio-posix: zero out io_uring sqe user_data

liburing does not clear sqe->user_data. We must do it ourselves to avoid
undefined behavior in process_cqe() when user_data is used.

Note that fdmon-io_uring is currently disabled, so this is a latent bug
that does not affect users. Let's merge this fix now to make it easier
to enable fdmon-io_uring in the future (and I'm working on that).

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230426212639.82310-1-stefanha@redhat.com>