ZTS: Optimize Kernel Same-page Merging (KSM)

Kernel same-page Merging (KSM) allows KVM guests to share identical
memory pages. These shared pages are usually common libraries or other
identical, high-use data.

The current configuration was a bit to lazy - so KSM didn't work very
well. With the new configuration I could run 3 Linux VMs in parralel.

FreeBSD can't benefit from it. But FreeBSD is not so memory hungry in
general, so there is no need for it ;)

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #16641