libnvdimm, label: populate 'isetcookie' for blk-aperture namespaces

Starting with the v1.2 definition of namespace labels, the isetcookie
field is populated and validated for blk-aperture namespaces. This adds
some safety against inadvertent copying of namespace labels from one
DIMM-device to another.

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