Intel describes the behavior of their processors as "undefined" if two or

Intel describes the behavior of their processors as "undefined" if two or
more mappings to the same physical page have different memory types, i.e.,
PAT settings.  Consequently, if pmap_change_attr() is applied to a virtual
address range within the kernel map, then the corresponding ranges of the
direct map also need to be changed.  Enhance pmap_change_attr() to handle
this case automatically.

Add a comment describing what pmap_change_attr() does.

Discussed with:	jhb