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>