mlxsw: spectrum_qdisc: Introduce struct mlxsw_sp_qdisc_state

In order to have a tidy structure where to put information related to Qdisc
offloads, introduce a new structure. Move there the two existing pieces of
data: root_qdisc and tclass_qdiscs. Embed them directly, because there's no
reason to go through pointer anymore. Convert users, update init/fini
functions.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>