pmap: Fix largemap restart checks in the kernel_maps sysctl handler

The purpose of these checks is to ensure that the address of the
next-level page table page is valid, since nothing is synchronizing with
a concurrent update of the large map and large map PTPs are freed to the
system. However, if PG_PS is set, there is no next level.

Reported by: rpokala
Reviewed by: kib
Tested by: rpokala
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28922

(cherry picked from commit aac25e222525780db8939d07a594d3e090c0a148)