Run fadvise with mmap file (#10142)

Run fadvise with mmap file (#10142)

Summary:
Right now with mmap file, we don't run fadvise following users' requests. There is no reason for that so this diff does that.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10142

Test Plan:
A simple readrandom against files with page cache dropped shows latency improvement from 7.8 us to 2.8:

./db_bench -use_existing_db --benchmarks=readrandom --num=100

Reviewed By: anand1976

Differential Revision: D37074975

fbshipit-source-id: ccc72bcac1b5fd634eb8fa2b6a5d9afe332e0bf6