devlink: Allow for devlink-rate nodes parent reassignment

Currently it's not possible to reassign the parent of the node using one
command. As the previous commit introduced a way to export entire
hierarchy from the driver, being able to modify and reassign parents
become important. This way user might easily change QoS settings without
interrupting traffic.

Example command:
devlink port function rate set pci/0000:4b:00.0/1 parent node_custom_1

This reassigns leaf node parent to node_custom_1.
Signed-off-by: Michal Wilczynski <michal.wilczynski@intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/devlink.c

@@ -1887,10 +1887,8 @@ devlink_nl_rate_parent_node_set(struct devlink_rate *devlink_rate,
 	int err = -EOPNOTSUPP;
 
 	parent = devlink_rate->parent;
-	if (parent && len) {
-		NL_SET_ERR_MSG_MOD(info->extack, "Rate object already has parent.");
-		return -EBUSY;
-	} else if (parent && !len) {
+
+	if (parent && !len) {
 		if (devlink_rate_is_leaf(devlink_rate))
 			err = ops->rate_leaf_parent_set(devlink_rate, NULL,
 							devlink_rate->priv, NULL,
@@ -1904,7 +1902,7 @@ devlink_nl_rate_parent_node_set(struct devlink_rate *devlink_rate,
 
 		refcount_dec(&parent->refcnt);
 		devlink_rate->parent = NULL;
-	} else if (!parent && len) {
+	} else if (len) {
 		parent = devlink_rate_node_get_by_name(devlink, parent_name);
 		if (IS_ERR(parent))
 			return -ENODEV;
@@ -1931,6 +1929,10 @@ devlink_nl_rate_parent_node_set(struct devlink_rate *devlink_rate,
 		if (err)
 			return err;
 
+		if (devlink_rate->parent)
+			/* we're reassigning to other parent in this case */
+			refcount_dec(&devlink_rate->parent->refcnt);
+
 		refcount_inc(&parent->refcnt);
 		devlink_rate->parent = parent;
 	}