Don't allow MAXMEM or hw.physmem to extend the top of memory if our memory
map was obtained from the SMAP. SMAP is trustworthy, and the memory
extending feature is a band-aid for older systems where FreeBSD's methods
of detecting memory were not always trustworthy. This fixes the issue
where using hw.physmem could result in the ACPI tables getting trashed
breaking ACPI.

MFC after: 3 days
Tested on: i386