x86/mm: avoid IOMMU operations in more cases in _get_page_type()

x86/mm: avoid IOMMU operations in more cases in _get_page_type()

All that really matters is whether writability of a page changes; in
particular e.g. page table -> page table (but different levels)
transitions do not require unmapping the page from the IOMMU again.

Note that the XSA-288 fix did arrange for PGT_none pages not needing
special consideration here.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>