libnvdimm, pfn, convert nd_pfn_probe() to devm

Pass the device performing the probe so we can use a devm allocation for
the pfn superblock.

Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>