libnvdimm, pfn: 'struct page' provider infrastructure

Implement the base infrastructure for libnvdimm PFN devices. Similar to
BTT devices they take a namespace as a backing device and layer
functionality on top. In this case the functionality is reserving space
for an array of 'struct page' entries to be handed out through
pfn_to_page(). For now this is just the basic libnvdimm-device-model for
configuring the base PFN device.

As the namespace claiming mechanism for PFN devices is mostly identical
to BTT devices drivers/nvdimm/claim.c is created to house the common
bits.

Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/nvdimm/region.c

@@ -53,6 +53,7 @@ static int nd_region_probe(struct device *dev)
 		return -ENODEV;
 
 	nd_region->btt_seed = nd_btt_create(nd_region);
+	nd_region->pfn_seed = nd_pfn_create(nd_region);
 	if (err == 0)
 		return 0;
 
@@ -84,6 +85,7 @@ static int nd_region_remove(struct device *dev)
 	nvdimm_bus_lock(dev);
 	nd_region->ns_seed = NULL;
 	nd_region->btt_seed = NULL;
+	nd_region->pfn_seed = NULL;
 	dev_set_drvdata(dev, NULL);
 	nvdimm_bus_unlock(dev);