libnvdimm, dax: fix alignment validation

Testing the dax-device autodetect support revealed a probe failure with
the following result:

    dax0.1: bad offset: 0x8200000 dax disabled

The original pfn-device implementation inferred the alignment from
ilog2(offset), now that the alignment is explicit the is_power_of_2()
needs replacing with a real sanity check against the recorded alignment.
Otherwise the alignment check is useless in the implicit case and only
the minimum size of the offset matters.

This self-consistency check is further validated by the probe path that
will re-check that the offset is large enough to contain all the
metadata required to enable the device.

Cc: <stable@vger.kernel.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/nvdimm/pfn_devs.c

@@ -416,6 +416,8 @@ int nd_pfn_validate(struct nd_pfn *nd_pfn, const char *sig)
 			return -ENODEV;
 	}
 
+	if (nd_pfn->align == 0)
+		nd_pfn->align = le32_to_cpu(pfn_sb->align);
 	if (nd_pfn->align > nvdimm_namespace_capacity(ndns)) {
 		dev_err(&nd_pfn->dev, "alignment: %lx exceeds capacity %llx\n",
 				nd_pfn->align, nvdimm_namespace_capacity(ndns));
@@ -436,8 +438,8 @@ int nd_pfn_validate(struct nd_pfn *nd_pfn, const char *sig)
 		return -EBUSY;
 	}
 
-	nd_pfn->align = le32_to_cpu(pfn_sb->align);
-	if (!is_power_of_2(offset) || offset < PAGE_SIZE) {
+	if ((nd_pfn->align && !IS_ALIGNED(offset, nd_pfn->align))
+			|| !IS_ALIGNED(offset, PAGE_SIZE)) {
 		dev_err(&nd_pfn->dev, "bad offset: %#llx dax disabled\n",
 				offset);
 		return -ENXIO;