If a superpage mapping is being removed then we need to ignore the PG_PDE_PAT

bit when looking up the vm_page associated with the superpage's physical
address.

If the caching attribute for the mapping is write combining or write protected
then the PG_PDE_PAT bit will be set and thus cause an 'off-by-one' error
when looking up the vm_page.

Fix this by using the PG_PS_FRAME mask to compute the physical address for
a superpage mapping instead of PG_FRAME.

This is a theoretical issue at this point since non-writeback attributes are
currently used only for fictitious mappings and fictitious mappings are not
subject to promotion.

Discussed with:	alc, kib
MFC after:	2 weeks

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