libnvdimm, pfn: fix ARCH=alpha allmodconfig build failure

I had relied on the kbuild robot for cross build coverage, however it
only builds alpha_defconfig.  Switch from HPAGE_SIZE to PMD_SIZE, which
is more widely defined.

Fixes: 658922e5 ("libnvdimm, pfn: fix memmap reservation sizing")
Cc: <stable@vger.kernel.org>
Reported-by: Guenter Roeck <guenter@roeck-us.net>
Tested-by: Guenter Roeck <guenter@roeck-us.net>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>