intel-iommu: Introduce domain_pfn_mapping()

... and use it in the trivial cases; the other callers want individual
(and bisectable) attention, since I screwed them up the first time...

Make the BUG_ON() happen on too-large virtual address rather than
physical address, too. That's the one we care about.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>