While the first change is a bug fix (for, admittedly, a case which
apparently hasn't occurred in practice, or else we would have had
bug reports), it already puts in place an instance of what the 2nd
patch is proposing for perhaps wider use.

1: make p2m_get_page_from_gfn() handle grant and shared cases better
2: aid the compiler in folding p2m_is_...()

Jan