tdb: Use posix_fallocate

tdb: Use posix_fallocate

This should be significantly faster than pwriting.

openbsd doesn't have posix_fallocate, so we do need the fallback. Also, it
might have weird failure modes, so we keep the old code in place except for
posix_fallocate returning success or ENOSPC.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Thu Aug 24 05:38:49 CEST 2017 on sn-devel-144