[RocksDB Performance Branch] Fix a regression bug of munmap

Summary:
Fix a stupid bug I just introduced in b59d4d5a5051263b4bfcef00913219ffe4654e42, which I didn't even mean to include.
GCC might remove the munmap.

Test Plan: Run it and make sure munmap succeeds

Reviewers: haobo, kailiu

Reviewed By: kailiu

CC: dhruba, reconnect.grayhat, leveldb

Differential Revision: https://reviews.facebook.net/D14361