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>

util/fdmon-io_uring.c

@@ -184,6 +184,7 @@ static void add_poll_remove_sqe(AioContext *ctx, AioHandler *node)
 #else
     io_uring_prep_poll_remove(sqe, node);
 #endif
+    io_uring_sqe_set_data(sqe, NULL);
 }
 
 /* Add a timeout that self-cancels when another cqe becomes ready */
@@ -197,6 +198,7 @@ static void add_timeout_sqe(AioContext *ctx, int64_t ns)
 
     sqe = get_sqe(ctx);
     io_uring_prep_timeout(sqe, &ts, 1, 0);
+    io_uring_sqe_set_data(sqe, NULL);
 }
 
 /* Add sqes from ctx->submit_list for submission */