mlxsw: spectrum_qdisc: Track children per qdisc

mlxsw currently allows a two-level structure of qdiscs: the root and
possibly a number of children. In order to support offloading more general
qdisc trees, introduce to struct mlxsw_sp_qdisc a pointer to child qdiscs.
Refer to the child qdiscs through this pointer, instead of going through
the tclass_qdiscs in qdisc_state. Additionally introduce a field
num_classes, which holds number of given qdisc's children.

Also introduce a generic function for walking qdisc trees. Rewrite
mlxsw_sp_qdisc_find() and _find_by_handle() to use the generic walker.

For now, keep the qdisc_state.tclass_qdisc, and just point root_qdiscs's
children to this array. Following patches will make the allocation dynamic.
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>