iommu/vtd: fix address translation for leaf entries

iommu/vtd: fix address translation for leaf entries

Fix two issues related to leaf address lookups in VT-d:

* When translating an address that falls inside of a superpage in the
  IOMMU page tables the fetching of the PTE value wasn't masking of the
  contiguous related data, which caused the returned data to be
  corrupt as it would contain bits that the caller would interpret as
  part of the address.

* When the requested leaf address wasn't mapped by a superpage the
  returned value wouldn't have any of the low 12 bits set, thus missing
  the permission bits expected by the caller.

Take the opportunity to also adjust the function comment to note that
when returning the full PTE the bits above PADDR_BITS are removed.

Fixes: c71e55501a61 ('VT-d: have callers specify the target level for page table walks')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>