Summary:
VerifyFileChecksums currently interprets the readahead_size as a payload of readahead_size for calculating the checksum, plus a prefetch of an additional readahead_size. Hence each read is readahead_size * 2. This change treats it as chunks of readahead_size for checksum calculation.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11328

Test Plan: Add a unit test

Reviewed By: pdillinger

Differential Revision: D44718781

Pulled By: anand1976

fbshipit-source-id: 79bae1ebaa27de2a13bc86f5910bf09356936e63