vm: Add a KVA arena for M_NEVERFREED allocations

This patch adds a new KVA arena for separating M_NEVERFREED allocations.
Separating KVAs for pages that are never freed should facilitate
superpage promotion in the kernel.

Differential Revision: https://reviews.freebsd.org/D45997
Reviewed by: alc, kib, markj
Tested by: alc