makefs: avoid uninitialized memory in root directory date

Move date setting into cd9660_populate_iso_dir_record so there is no
path that leaves it unset.

PR: 203531
Reported by: Thomas Schmitt <scdbackup@gmx.net>
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39258