vfio: use static window sizing for sPAPR IOMMU

vfio: use static window sizing for sPAPR IOMMU

The SPAPR IOMMU requires that a DMA window size be defined before memory
can be mapped for DMA. Current code dynamically modifies the DMA window
size in response to every new memory allocation which is potentially
dangerous because all existing mappings need to be unmapped/remapped in
order to resize the DMA window, leaving hardware holding IOVA addresses
that are temporarily unmapped.  The new SPAPR code statically assigns
the DMA window size on first use, using the largest physical memory
memory address when IOVA=PA and the highest existing memseg virtual
address when IOVA=VA.

Signed-off-by: David Christensen <drc@linux.vnet.ibm.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>