Summary:
see https://github.com/facebook/rocksdb/issues/977; there are issues
with fallocate() on certain filesystems/kernel versions that can lead it to pre-
allocating blocks but never freeing them, even if they're unused.

Test Plan:
verified build commands omit DROCKSDB_FALLOCATE_PRESENT when this env
variable is set.

without disabling it:

  $ ROCKSDB_NO_FBCODE=1 make -n env_test | grep -q DROCKSDB_FALLOCATE_PRESENT ; echo $?
  0

with disabling it:

  $ ROCKSDB_NO_FBCODE=1 DISABLE_FALLOCATE=1 make -n env_test | grep -q DROCKSDB_FALLOCATE_PRESENT ; echo $?
  1

Reviewers: kradhakrishnan, yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D54069