Fix use_after_free bug when underlying FS enables kFSBuffer (#11645)

Fix use_after_free bug when underlying FS enables kFSBuffer (#11645)

Summary:
Fix use_after_free bug in async_io MultiReads when underlying FS enabled kFSBuffer. kFSBuffer is when underlying FS pass their own buffer instead of using RocksDB scratch in FSReadRequest
Since it's an experimental feature, added a hack for now to fix the bug.
Planning to make public API change to remove const from the callback as it doesn't make sense to use const.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11645

Test Plan: tested locally

Reviewed By: ltamasi

Differential Revision: D47819907

Pulled By: akankshamahajan15

fbshipit-source-id: 1faf5ef795bf27e2b3a60960374d91274931df8d