net/sched: sch_api: conditional netlink notifications

Implement conditional netlink notifications for Qdiscs and classes,
which were missing in the initial patches that targeted tc filters and
actions. Notifications will only be built after passing a check for
'rtnl_notify_needed()'.

For both Qdiscs and classes 'get' operations now call a dedicated
notification function as it was not possible to distinguish between
'create' and 'get' before. This distinction is necessary because 'get'
always send a notification.

Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://lore.kernel.org/r/20231229132642.1489088-2-pctammela@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>