Fix build on 13-CURRENT >= 1300062

Changes to the virtual memory subsystem in 13.0-CURRENT r355491 has changed
the API, as it is no longer a circular array. This broke the devel/libgtop
port on these systems.

Submitted by: Neel Chauhan <neel AT neelc DOT org> (based on patch by dougm)
Reviewed by: koobs
Differential Revision: https://reviews.freebsd.org/D22929