iommupt: Add unmap_pages op

unmap_pages removes mappings and any fully contained interior tables from
the given range. This follows the now-standard iommu_domain API definition
where it does not split up larger page sizes into smaller. The caller must
perform unmap only on ranges created by map or it must have somehow
otherwise determined safe cut points (eg iommufd/vfio use iova_to_phys to
scan for them)

A future work will provide 'cut' which explicitly does the page size split
if the HW can support it.

unmap is implemented with a recursive descent of the tree. If the caller
provides a VA range that spans an entire table item then the table memory
can be freed as well.

If an entire table item can be freed then this version will also check the
leaf-only level of the tree to ensure that all entries are present to
generate -EINVAL. Many of the existing drivers don't do this extra check.

This version sits under the iommu_domain_ops as unmap_pages() but does not
require the external page size calculation. The implementation is actually
unmap_range() and can do arbitrary ranges, internally handling all the
validation and supporting any arrangment of page sizes. A future series
can optimize __iommu_unmap() to take advantage of this.

Freed page table memory is batched up in the gather and will be freed in
the driver's iotlb_sync() callback after the IOTLB flush completes.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Samiullah Khawaja <skhawaja@google.com>
Tested-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Tested-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>