Summary:
```TestFSWritableFile``` buffers data in ```Append``` in order to simulate unsynced data loss on crash. This is only required for buffered IO and should be disabled for direct IO. Otherwise, it causes crash tests to assert on the buffer address alignment - ```db_stress: env/io_posix.cc:1194: virtual rocksdb::IOStatus rocksdb::PosixWritableFile::Append(const rocksdb::Slice&, const rocksdb::IOOptions&, rocksdb::IODebugContext*): Assertion `IsSectorAligned(data.data(), GetRequiredBufferAlignment())' failed.```.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/8490
Reviewed By: zhichao-cao
Differential Revision: D29565080
Pulled By: anand1976
fbshipit-source-id: 682831fd66ed3b9597caa74fc453e22dfaf9b973
df4197ca6e