iommufd/selftest: Cover domain unmap with huge pages and access

Inspired by the syzkaller reproducer check the batch carry path with a
simple test.

Link: https://lore.kernel.org/r/4-v1-ceab6a4d7d7a+94-iommufd_syz_jgg@nvidia.com


Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>