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: 658922e57b ("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>

drivers/nvdimm/pmem.c

@@ -402,9 +402,9 @@ static int nd_pfn_init(struct nd_pfn *nd_pfn)
 
 		/*
 		 * vmemmap_populate_hugepages() allocates the memmap array in
-		 * HPAGE_SIZE chunks.
+		 * PMD_SIZE chunks.
 		 */
-		memmap_size = ALIGN(64 * npfns, HPAGE_SIZE);
+		memmap_size = ALIGN(64 * npfns, PMD_SIZE);
 		offset = ALIGN(start + SZ_8K + memmap_size, nd_pfn->align)
 			- start;
 	} else if (nd_pfn->mode == PFN_MODE_RAM)