net: sched: provide notification for graft on root

Drivers are currently not notified when a Qdisc is grafted as root.
This requires special casing Qdiscs added with parent = TC_H_ROOT in
the driver.  Also there is no notification sent to the driver when
an existing Qdisc is grafted as root.

Add this very simple notifications, drivers should now be able to
track their Qdisc tree fully.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>