devlink: Allow setting parent node of rate objects

Refactor DEVLINK_CMD_RATE_{GET|SET} command handlers to support setting
a node as a parent for another rate object (leaf or node) by means of
new attribute DEVLINK_ATTR_RATE_PARENT_NODE_NAME. Extend devlink ops
with new callbacks rate_{leaf|node}_parent_set() to set node as a parent
for rate object to allow supporting drivers to implement rate grouping
through devlink. Driver implementations are allowed to support leafs
or node children only. Invoking callback with NULL as parent should be
threated by the driver as unset parent action.
Extend rate object struct with reference counter to disallow deleting a
node with any child pointing to it. User should unset parent for the
child explicitly.

Example:

$ devlink port function rate add netdevsim/netdevsim10/group1

$ devlink port function rate add netdevsim/netdevsim10/group2

$ devlink port function rate set netdevsim/netdevsim10/group1 parent group2

$ devlink port function rate show netdevsim/netdevsim10/group1
netdevsim/netdevsim10/group1: type node parent group2

$ devlink port function rate set netdevsim/netdevsim10/group1 noparent

Co-developed-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Dmytro Linkin <dlinkin@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>