Fix a bug introduced in r252646 that causes a page with the PG_PTE_PAT bit set
to be interpreted as a superpage. This is because PG_PTE_PAT is at the same
bit position in PTE as PG_PS is in a PDE.

This caused a number of regressions on amd64 systems: panic when starting
X applications, freeze during shutdown etc.

Pointy hat to:	me
Tested by: gperez@entel.upc.edu, joel, dumbbell
Reviewed by: kib