Truncate unused space on PosixWritableFile::Close()

Summary:
Blocks allocated with fallocate will take extra space on disk even if they are unused and the file is close.

Now we remove the extra blocks at the end of the file by calling `ftruncate`.

Test Plan: added a test to env_test

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16647