Fix Allocate test

Summary: For some reason, on a subset of our continuous build machines, preallocation is allocating 8 block more than it should be. Let's relax the test a little bit -- now we require the test to allocate *at least* the number of blocks as we told them to.

Test Plan: no

Reviewers: ljin, haobo, sdong

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18141