libnvdimm, namespace: unify blk and pmem label scanning

In preparation for allowing multiple namespace per pmem region, unify
blk and pmem label scanning.  Given that blk regions already support
multiple namespaces, teaching that path how to do pmem namespace
scanning is an incremental step towards multiple pmem namespace support.
This should be functionally equivalent to the previous state in that
stops after finding the first valid pmem label set.
Signed-off-by: Dan Williams <dan.j.williams@intel.com>