libnvdimm, namespace: update label implementation for multi-pmem

Instead of assuming that there will only ever be one allocated range at
the start of the region, account for additional namespaces that might
start at an offset from the region base.

After this change pmem namespaces now have a reason to carry an array of
resources similar to blk.  Unifying the resource tracking infrastructure
in nd_namespace_common is a future cleanup candidate.

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