libnvdimm, pfn: fix align attribute

Fix the format specifier so that the attribute can be parsed correctly.
Currently it returns decimal 1000 for a 4096-byte alignment.

Cc: <stable@vger.kernel.org>
Reported-by: Dave Jiang <dave.jiang@intel.com>
Fixes: 315c5625

 ("libnvdimm, pfn: add 'align' attribute, default to HPAGE_SIZE")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>