device-dax, tools/testing/nvdimm: enable device-dax with mock resources

Provide a replacement pgoff_to_phys() that translates an nfit_test
resource (allocated by vmalloc()) to a pfn.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>