mlxsw: spectrum_qdisc: Query tclass / priomap instead of caching it

Currently when keeping track of qdiscs, mlxsw notes the TC and priomap
corresponding to each qdisc. That is fine currently, as there only ever is
one level of qdiscs to update: the direct children of ETS / PRIO. However
as deeper structures are made offloadable, ETS would need to update these
values for the complete subtree, and interim qdiscs would need to remember
to propagate the value.

Instead, reverse the responsibility: child qdiscs can ask their parent what
their TC and priomap are. ETS / PRIO know the answer right away, or there
are defaults for when the root qdisc does not assign them (e.g. when RED is
used as root qdisc). When RED and TBF become classful, they will simply
forward the request up to their parent.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>