[3.6.fb] use fallocate(FALLOC_FL_PUNCH_HOLE) to release unused blocks at the end...

[3.6.fb] use fallocate(FALLOC_FL_PUNCH_HOLE) to release unused blocks at the end of file

Summary:
ftruncate does not always free preallocated unused space at the end of file.
In some cases, we pin too much disk space than it should
This is a fix previously reviewed in on https://reviews.facebook.net/D25641

Test Plan: env_test

Reviewers: igor, ljin, sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D27975