filejournal: add journal pre-allocate for osx

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
Merge pull request #837 from ceph/port/fallocate

FileJournal: zero-fill in-lieu of posix_fallocate

We may want to change that to a #warning later...

Reviewed-by: Sage Weil <sage@inktank.com>