Disable vm map consistency checking by default on INVARIANTS kernels.

The checks are too expensive for a general-purpose kernel. Enable the
checks when DIAGNOSTIC is defined and provide a sysctl to enable the
checks in a non-DIAGNOSTIC INVARIANTS kernel.

Reviewed by: kib
Discussed with: Doug Moore <dougm@rice.edu>
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19999