Revert "arm: Bump KSTACK_PAGES default to match i386/amd64"

This reverts commit b684d812fcb04b2997fd755405a92c36b9f6e30e.

It causes an issue on a pfsense routing workload where memory
fragmentation prevents the necessary consecutive pages from being
readily available.

Reported by: pfsense (mjg, scottl)
Approved by: ian
MFC after: 1 day
Differential Revision: https://reviews.freebsd.org/D31244