io_uring: reuse io_req_task_queue_fail()

Use io_req_task_queue_fail() on the fail path of io_req_task_queue().
It's unlikely to happen, so don't care about additional overhead, but
allows to keep all the req->result invariant in a single function.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>