io_uring: don't do opcode prep twice

Calling into opcode prep handlers may be dangerous, as they re-read
SQE but might not re-initialise requests completely. If io_req_defer()
passed fast checks and is done with preparations, punt it async.

As all other cases are covered with nulling @sqe, this guarantees that
io_[opcode]_prep() are visited only once per request.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -5447,7 +5447,8 @@ static int io_req_defer(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 	if (!req_need_defer(req, seq) && list_empty(&ctx->defer_list)) {
 		spin_unlock_irq(&ctx->completion_lock);
 		kfree(de);
-		return 0;
+		io_queue_async_work(req);
+		return -EIOCBQUEUED;
 	}
 
 	trace_io_uring_defer(ctx, req, req->user_data);