Make vm_map_max/min/pmap KBI stable.

There are out of tree consumers of vm_map_min() and vm_map_max(), and
I believe there are consumers of vm_map_pmap(), although the later is
arguably less in the need of KBI-stable interface. For the consumers
benefit, make modules using this KPI not depended on the struct vm_map
layout.

Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D14902