amd64/pmap: unbreak !NUMA case for fictitious pages

A fictitious page can have a physical address beyond the end of the RAM.
In the NUMA case there is some special code to handle such pages, but in
the other case the pages are handled the same as normal pages. So, we
cannot assert that the physical address is within RAM addresses.

Suggested by: kib
Reviewed by: kib
X-MFC note: NUMA support has not been MFC-ed