iommu/vt-d: Reduce extra first level entry in iommu->domains

In commit <8bf47816> ("iommu/vt-d: Split up iommu->domains array"), it
it splits iommu->domains in two levels. Each first level contains 256
entries of second level. In case of the ndomains is exact a multiple of
256, it would have one more extra first level entry for current
implementation.

This patch refines this calculation to reduce the extra first level entry.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>