io_uring: Fix corrupted user_data

There is a bug, where failed linked requests are returned not with
specified @user_data, but with garbage from a kernel stack.

The reason is that io_fail_links() uses req->user_data, which is
uninitialised when called from io_queue_sqe() on fail path.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2448,6 +2448,8 @@ static void io_submit_sqe(struct io_ring_ctx *ctx, struct sqe_submit *s,
 		return;
 	}
 
+	req->user_data = s->sqe->user_data;
+
 	/*
 	 * If we already have a head request, queue this one for async
 	 * submittal once the head completes. If we don't have a head but