io_uring: ensure async buffered read-retry is setup properly

ANBZ: torvalds#190

commit f38c7e3 upstream.

A previous commit for fixing up short reads botched the async retry
path, so we ended up going to worker threads more often than we should.
Fix this up, so retries work the way they originally were intended to.

Fixes: 227c0c9 ("io_uring: internally retry short reads")
Reported-by: Hao_Xu <haoxu@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Reviewed-by: Hao Xu <haoxu@linux.alibaba.com>
Tested-by: Hao Xu <haoxu@linux.alibaba.com>

fs/io_uring.c

@@ -3101,10 +3101,8 @@ static int io_read(struct io_kiocb *req, bool force_nonblock)
 			goto done;
 		/* some cases will consume bytes even on error returns */
 		iov_iter_revert(iter, iov_count - iov_iter_count(iter));
-		ret = io_setup_async_rw(req, iovec, inline_vecs, iter, false);
-		if (ret)
-			goto out_free;
-		return -EAGAIN;
+		ret = 0;
+		goto copy_iov;
 	} else if (ret < 0) {
 		goto out_free;
 	}