iommufd: IOCTLs for the io_pagetable

Connect the IOAS to its IOCTL interface. This exposes most of the
functionality in the io_pagetable to userspace.

This is intended to be the core of the generic interface that IOMMUFD will
provide. Every IOMMU driver should be able to implement an iommu_domain
that is compatible with this generic mechanism.

It is also designed to be easy to use for simple non virtual machine
monitor users, like DPDK:
 - Universal simple support for all IOMMUs (no PPC special path)
 - An IOVA allocator that considers the aperture and the allowed/reserved
   ranges
 - io_pagetable allows any number of iommu_domains to be connected to the
   IOAS
 - Automatic allocation and re-use of iommu_domains

Along with room in the design to add non-generic features to cater to
specific HW functionality.

Link: https://lore.kernel.org/r/11-v6-a196d26f289e+11787-iommufd_jgg@nvidia.com
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Yi Liu <yi.l.liu@intel.com>
Tested-by: Lixiao Yang <lixiao.yang@intel.com>
Tested-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>