mlxsw: spectrum_qdisc: Guard all qdisc accesses with a lock

The FIFO handler currently guards accesses to the future FIFO tracking by
asserting RTNL. In the future, the changes to the qdisc state will be more
thorough, so other qdiscs will need this guarding is as well. In order
to not further the RTNL infestation, instead convert to a custom lock that
will guard accesses to the qdisc state.

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