ZTS: Optimize KSM on Linux and remove it for FreeBSD

Don't use KSM on the FreeBSD VMs and optimize KSM settings for
Linux to have faster run times.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #17247