nfit, libnvdimm, region: export 'position' in mapping info

It is useful to be able to know the position of a DIMM in an
interleave-set. Consider the case where the order of the DIMMs changes
causing a namespace to be invalidated because the interleave-set cookie no
longer matches. If the before and after state of each DIMM position is
known this state debugged by the system owner.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/include/linux/libnvdimm.h b/include/linux/libnvdimm.h
index f3d3e6a..9b8d81a 100644
--- a/include/linux/libnvdimm.h
+++ b/include/linux/libnvdimm.h
@@ -87,6 +87,7 @@ struct nd_mapping_desc {
 	struct nvdimm *nvdimm;
 	u64 start;
 	u64 size;
+	int position;
 };
 
 struct nd_region_desc {