io_uring: fix failed linkchain code logic

Given a linkchain like this:
req0(link_flag)-->req1(link_flag)-->...-->reqn(no link_flag)

There is a problem:
 - if some intermediate linked req like req1 's submittion fails, reqs
   after it won't be cancelled.

   - sqpoll disabled: maybe it's ok since users can get the error info
     of req1 and stop submitting the following sqes.

   - sqpoll enabled: definitely a problem, the following sqes will be
     submitted in the next round.

The solution is to refactor the code logic to:
 - if a linked req's submittion fails, just mark it and the head(if it
   exists) as REQ_F_FAIL. Leverage req->result to indicate whether it
   is failed or cancelled.
 - submit or fail the whole chain when we come to the end of it.

Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/20210827094609.36052-3-haoxu@linux.alibaba.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>