device-dax: fix cdev leak

If device_add() fails, cleanup the cdev. Otherwise, we leak a kobj_map()
with a stale device number.

As Jason points out, there is a small possibility that userspace has
opened and mapped the device in the time between cdev_add() and the
device_add() failure. We need a new kill_dax_dev() helper to invalidate
any established mappings.

Fixes: ba09c01d2fa8 ("dax: convert to the cdev api")
Cc: <[e-mail]>
Reported-by: Jason Gunthorpe <[e-mail]>
Signed-off-by: Dan Williams <[e-mail]>
Signed-off-by: Logan Gunthorpe <[e-mail]>
Reviewed-by: Johannes Thumshirn <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/dax/dax.c

···
 	kfree(dax_dev);
 }
 
-static void unregister_dax_dev(void *dev)
+static void kill_dax_dev(struct dax_dev *dax_dev)
 {
-	struct dax_dev *dax_dev = to_dax_dev(dev);
 	struct cdev *cdev = &dax_dev->cdev;
-
-	dev_dbg(dev, "%s\n", __func__);
 
 	/*
 	 * Note, rcu is not protecting the liveness of dax_dev, rcu is
···
 	synchronize_rcu();
 	unmap_mapping_range(dax_dev->inode->i_mapping, 0, 0, 1);
 	cdev_del(cdev);
+}
+
+static void unregister_dax_dev(void *dev)
+{
+	struct dax_dev *dax_dev = to_dax_dev(dev);
+
+	dev_dbg(dev, "%s\n", __func__);
+
+	kill_dax_dev(dax_dev);
 	device_unregister(dev);
 }
 
···
 	dev_set_name(dev, "dax%d.%d", dax_region->id, dax_dev->id);
 	rc = device_add(dev);
 	if (rc) {
+		kill_dax_dev(dax_dev);
 		put_device(dev);
 		return ERR_PTR(rc);
 	}