Summary:
We had a crash in this code: `fstat()` failed; `file_stats` contained garbage, in particular `file_stats.st_blksize == 6`; the expression `file_stats.st_blocks / (file_stats.st_blksize / 512)` divided by zero.
Closes https://github.com/facebook/rocksdb/pull/2420
Differential Revision: D5216110
Pulled By: al13n321
fbshipit-source-id: 6d8fc5e7c4f98c1139e68c7829ebdbac68b0fce0