mm/hmm: add tests for hmm_pfn_to_map_order()

Add a sanity test for hmm_range_fault() returning the page mapping size
order.

Link: https://lore.kernel.org/r/20200701225352.9649-6-rcampbell@nvidia.com


Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>