Fix reseting of async_read_in_progress_ variable in FilePrefetchBuffer to call Poll...

Fix reseting of async_read_in_progress_ variable in FilePrefetchBuffer to call Poll API (#9815)

Summary:
Currently RocksDB reset async_read_in_progress_ in callback
due to which underlying filesystem relying on Poll API won't be called
leading to stale memory access.
In order to fix it, async_read_in_progress_ will be reset after Poll API
is called to make sure underlying file_system waiting on Poll can clear
its state or take appropriate action.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9815

Test Plan: CircleCI tests

Reviewed By: anand1976

Differential Revision: D35451534

Pulled By: akankshamahajan15

fbshipit-source-id: b70ef6251a7aa9ed4876ba5e5100baa33d7d474c