FreeBSD: use zero_region instead of allocating a dedicated page

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Closes #13406