libnvdimm: Export the target_node attribute for regions and namespaces

Aneesh points out that some platforms may have "local" attached
persistent memory and "remote" persistent memory that map to the same
"online" node, or persistent memory devices with different performance
properties. In this case 'numa_node' is identical for the two instances,
but 'target_node' is differentiated so platform firmware can communicate
distinct performance properties per range. Expose 'target_node' by
default to allow for disambiguation of devices that share the same
numa_map_to_online_node() result.

Reported-by: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Link: https://lore.kernel.org/r/157401274500.43284.2369509941678577768.stgit@dwillia2-desk3.amr.corp.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>