nvdimm: rectify the illogical code within nd_dax_probe()

When nd_dax is NULL, nd_pfn is consequently NULL as well. Nevertheless,
it is inadvisable to perform pointer arithmetic or address-taking on a
NULL pointer.
Introduce the nd_dax_devinit() function to enhance the code's logic and
improve its readability.

Signed-off-by: Yi Yang <yiyang13@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20241108085526.527957-1-yiyang13@huawei.com
Signed-off-by: Ira Weiny <ira.weiny@intel.com>

Author: Yi Yang

drivers/nvdimm/dax_devs.c

@@ -106,12 +106,12 @@ int nd_dax_probe(struct device *dev, struct nd_namespace_common *ndns)
 
 	nvdimm_bus_lock(&ndns->dev);
 	nd_dax = nd_dax_alloc(nd_region);
-	nd_pfn = &nd_dax->nd_pfn;
-	dax_dev = nd_pfn_devinit(nd_pfn, ndns);
+	dax_dev = nd_dax_devinit(nd_dax, ndns);
 	nvdimm_bus_unlock(&ndns->dev);
 	if (!dax_dev)
 		return -ENOMEM;
 	pfn_sb = devm_kmalloc(dev, sizeof(*pfn_sb), GFP_KERNEL);
+	nd_pfn = &nd_dax->nd_pfn;
 	nd_pfn->pfn_sb = pfn_sb;
 	rc = nd_pfn_validate(nd_pfn, DAX_SIG);
 	dev_dbg(dev, "dax: %s\n", rc == 0 ? dev_name(dax_dev) : "<none>");

drivers/nvdimm/nd.h

@@ -600,6 +600,13 @@ struct nd_dax *to_nd_dax(struct device *dev);
 int nd_dax_probe(struct device *dev, struct nd_namespace_common *ndns);
 bool is_nd_dax(const struct device *dev);
 struct device *nd_dax_create(struct nd_region *nd_region);
+static inline struct device *nd_dax_devinit(struct nd_dax *nd_dax,
+					    struct nd_namespace_common *ndns)
+{
+	if (!nd_dax)
+		return NULL;
+	return nd_pfn_devinit(&nd_dax->nd_pfn, ndns);
+}
 #else
 static inline int nd_dax_probe(struct device *dev,
 		struct nd_namespace_common *ndns)