fsync directory after creating current file in NewDB()

Summary: One of our users reported current file corruption. The machine was rebooted during the time. This is the only think I can think of which could cause current file corruption. Just add this paranoid check.

Test Plan: make all check

Reviewers: haobo, igor

Reviewed By: haobo

CC: yhchiang, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D18495