Use a large kmem arena import size on NUMA systems.

This helps minimize internal fragmentation that occurs when 2MB imports
are interleaved across NUMA domains. Virtually all KVA allocations on
direct map platforms consume more than one page, so the fragmentation
manifests as runs of 511 4KB page mappings in the kernel.

Reviewed by: alc, kib
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26050