dm: only initialize md->dax_dev if CONFIG_DAX_DRIVER is enabled

md->dax_dev defaults to NULL and there is no need to initialize it
if CONFIG_DAX_DRIVER is disabled.

Signed-off-by: Peng Wang <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm.c

···
 static struct mapped_device *alloc_dev(int minor)
 {
 	int r, numa_node_id = dm_get_numa_node();
-	struct dax_device *dax_dev = NULL;
 	struct mapped_device *md;
 	void *old_md;
 
···
 	sprintf(md->disk->disk_name, "dm-%d", minor);
 
 	if (IS_ENABLED(CONFIG_DAX_DRIVER)) {
-		dax_dev = alloc_dax(md, md->disk->disk_name, &dm_dax_ops);
-		if (!dax_dev)
+		md->dax_dev = alloc_dax(md, md->disk->disk_name, &dm_dax_ops);
+		if (!md->dax_dev)
 			goto bad;
 	}
-	md->dax_dev = dax_dev;
 
 	add_disk_no_queue_reg(md->disk);
 	format_dev_t(md->name, MKDEV(_major, minor));