libnvdimm/pfn_dev: Add page size and struct page size to pfn superblock

This is needed so that pmem probe don't wrongly initialize a namespace
which doesn't have enough space reserved for holding struct pages
with the current kernel.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Link: https://lore.kernel.org/r/20190905154603.10349-5-aneesh.kumar@linux.ibm.com


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