FileJournal: fix posix_fallocate error handling

From the man page for posix_fallocate:

    posix_fallocate() returns zero on success, or an error
    number on failure.  Note that errno is not set.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
Reviewed-by: Sage Weil <sage@inktank.com>