arm: Bump KSTACK_PAGES default to match i386/amd64

See 3f6867ef6386 for additional context.

It is also needed for OpenZFS performance and stability.

Reviewed by: ian (arm), imp
Differential Revision: https://reviews.freebsd.org/D31244

(cherry picked from commit b684d812fcb04b2997fd755405a92c36b9f6e30e)