VT-d: fix reduced page table levels support when sharing tables

VT-d: fix reduced page table levels support when sharing tables

domain_pgd_maddr() contains logic to adjust the root address to be put
in the context entry in case 4-level page tables aren't supported by an
IOMMU. This logic may not be bypassed when sharing page tables.

This is CVE-2021-28710 / XSA-390.

Fixes: 25ccd093425c ("iommu: remove the share_p2m operation")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>