VT-d: limit page table population in domain_pgd_maddr()

VT-d: limit page table population in domain_pgd_maddr()

I have to admit that I never understood why domain_pgd_maddr() wants to
populate all page table levels for DFN 0. I can only assume that despite
the comment there what is needed is population just down to the smallest
possible nr_pt_levels that the loop later in the function may need to
run to. Hence what is needed is the minimum of all possible
iommu->nr_pt_levels, to then be passed into addr_to_dma_page_maddr()
instead of literal 1.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>