nvdimm: Read NVDIMM namespace labels

When attaching to NVDIMM devices, read and verify the namespace
labels from the special namespace label storage area. A later
change will expose NVDIMM namespaces derived from this label data.

Submitted by: D Scott Phillips <d.scott.phillips@intel.com>
Discussed with: kib
MFC after: 1 week
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D18735