dax: add a facility to lookup a dax device by 'host' device name

For the current block_device based filesystem-dax path, we need a way
for it to lookup the dax_device associated with a block_device. Add a
'host' property of a dax_device that can be used for this purpose. It is
a free form string, but for a dax_device associated with a block device
it is the bdev name.

This is a stop-gap until filesystems are able to mount on a dax-inode
directly.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dax/device.c

@@ -645,7 +645,7 @@ struct dev_dax *devm_create_dev_dax(struct dax_region *dax_region,
 		goto err_id;
 	}
 
-	dax_dev = alloc_dax(dev_dax);
+	dax_dev = alloc_dax(dev_dax, NULL);
 	if (!dax_dev)
 		goto err_dax;