devlink: Introduce rate nodes

Implement support for DEVLINK_CMD_RATE_{NEW|DEL} commands that are used
to create and delete devlink rate nodes. Add new attribute
DEVLINK_ATTR_RATE_NODE_NAME that specify node name string. The node name
is an alphanumeric identifier. No valid node name can be a devlink port
index, eg. decimal number. Extend devlink ops with new callbacks
rate_node_{new|del}() and rate_node_tx_{share|max}_set() to allow
supporting drivers to implement ports rate grouping and setting tx rate
of rate nodes through devlink.
Expose devlink_rate_nodes_destroy() function to allow vendor driver do
proper cleanup of internally allocated resources for the nodes if the
driver goes down or due to any other reasons which requires nodes to be
destroyed.
Disallow moving device from switchdev to legacy mode if any node exists
on that device. User must explicitly delete nodes before switching mode.

Example:

$ devlink port function rate add netdevsim/netdevsim10/group1

$ devlink port function rate set netdevsim/netdevsim10/group1 \
        tx_share 10mbit tx_max 100mbit

Add + set command can be combined:

$ devlink port function rate add netdevsim/netdevsim10/group1 \
        tx_share 10mbit tx_max 100mbit

$ devlink port function rate show netdevsim/netdevsim10/group1
netdevsim/netdevsim10/group1: type node tx_share 10mbit tx_max 100mbit

$ devlink port function rate del netdevsim/netdevsim10/group1

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>