libnvdimm, region: move region-mapping input-paramters to nd_mapping_desc

Before we add more libnvdimm-private fields to nd_mapping make it clear
which parameters are input vs libnvdimm internals. Use struct
nd_mapping_desc instead of struct nd_mapping in nd_region_desc and make
struct nd_mapping private to libnvdimm.

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