IO prefetch is invalid

[kaede11]

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Expected behavior

when we set the options readahead_size = 512 * 1024, the expected behavior is to retrieve data from the prefetch buffer

Actual behavior

In function RandomAccessFileReader::ReadAsyncCallback, when use_direct_io() == false, the req.result has not been set appropriately

The data in the prefetch buf is not valid, as the value of the FilePrefetchBuffer::bufs_[index].buffer_.CurrentSize() has not been appropriately set when io callback in FilePrefetchBuffer::PrefetchAsyncCallback. As a result, it was read directly from the file again.

Steps to reproduce the behavior