amd64 sysctl vm.pmap.kernel_maps: do not desinformate for la57 case

For !la57, direct map, large map, and recursive mapping are not in top
pml4 region.

Bigger change would be adding the dump of whole pml5 KVA mappings.

Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D51364