Prepare the pmem driver to consume PMEM namespaces emitted by regions of
an nvdimm_bus instance.  No functional change.
Acked-by: NChristoph Hellwig <hch@lst.de>
Tested-by: NToshi Kani <toshi.kani@hp.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>