unix.c: use posix_fallocate() (#409)

Using of posix_fallocate() guarantees that, if it succeed, the
attempting to write to allocated space range does not fail because of
lack of storage space. This prevents SIGBUS when trying to write to
mmaped file and no space left.

Co-Authored-by: Ivan Zakharyaschev <imz@altlinux.org>
Reported-by: Mikhail Kulagin <m.kulagin at postgrespro dot ru>

Co-authored-by: Ivan Zakharyaschev <imz@altlinux.org>