x86: add an extra check when validating a huge pv L2 entry

x86: add an extra check when validating a huge pv L2 entry

While get_page_and_type_from_pagenr() (through get_page_from_pagenr())
does the needed mfn_valid() check, get_data_page() doesn't and, it
being passed a struct page_info pointer, really expects it's caller(s)
to do.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>