iommufd: File descriptor, context, kconfig and makefiles

This is the basic infrastructure of a new miscdevice to hold the iommufd
IOCTL API.

It provides:
 - A miscdevice to create file descriptors to run the IOCTL interface over

 - A table based ioctl dispatch and centralized extendable pre-validation
   step

 - An xarray mapping userspace ID's to kernel objects. The design has
   multiple inter-related objects held within in a single IOMMUFD fd

 - A simple usage count to build a graph of object relations and protect
   against hostile userspace racing ioctls

The only IOCTL provided in this patch is the generic 'destroy any object
by handle' operation.

Link: https://lore.kernel.org/r/6-v6-a196d26f289e+11787-iommufd_jgg@nvidia.com
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.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>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>