mlxsw: spectrum: qdiscs: Update backlog handling of a child qdiscs

When removing a child qdisc its backlog will be decreased from the parent
backlog. The driver backlog count should do the same.
When the parent changes its configuration, the child might need to clean
its stats. However, the backlog can't be cleaned with the rest of the
stats, because it reflects a momentary value that needs to be synced with
the core, not the history of the qdisc.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Reviewed-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>