Summary: https://reviews.facebook.net/D42321 has left PosixMmapFile in some weird state. This diff removes pending_sync_ that was now unused, fixes indentation and prevents Fsync() from calling both fsync() and fdatasync().

Test Plan: `make -j check`

Reviewers: sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D42885