fix(core): disable directory fsync on AIX/os400

[ThePrez]

Pretty self-explanatory change. Running this on IBM i (an AIX variant), I get:

Error: Failed to flush to storage {
  errorOnFsync: [Error: EINVAL: invalid argument, fsync] {
    errno: -22,
    code: 'EINVAL',
    syscall: 'fsync'
  },
  errorOnClose: null
}

AIX and IBM i ('os400') need the special case as Windows.

[wangshijun]

@ThePrez thanks for the fix, please run make bump-version in the repo root:

• bump version to 2.1.6
• update changelog accordingly

Then we can merge and release after ci pass

[ThePrez]

Sorry for the delay, @wangshijun, forced push with bumped version

[wangshijun]

@ThePrez this pr contains a lint error that needs to be fixed before merge