Summary:
`sync_file_range` returns `ENOSYS` on Windows Subsystem for Linux even
when using a supposedly supported filesystem like ext4. To handle this
case we can do a dynamic check that a no-op `sync_file_range`
invocation, which is accomplished by passing zero for the `flags`
argument, succeeds.

Also I rearranged the function and comments to hopefully make it more
easily understandable.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5416

Differential Revision: D15807061

fbshipit-source-id: d31d94e1f228b7850ea500e6199f8b5daf8cfbd3