Unfortunately, the workarounds for the hardware bugs make it pointless
to keep fine-grained locking for the MAC Merge state of each port.

Our vsc9959_cut_through_fwd() implementation requires
ocelot->fwd_domain_lock to be held, in order to serialize with changes
to the bridging domains and to port speed changes (which affect which
ports can be cut-through). Simultaneously, the traffic classes which can
be cut-through cannot be preemptible at the same time, and this will
depend on the MAC Merge layer state (which changes from threaded
interrupt context).

Since vsc9959_cut_through_fwd() would have to hold the mm->lock of all
ports for a correct and race-free implementation with respect to
ocelot_mm_irq(), in practice it means that any time a port's mm->lock is
held, it would potentially block holders of ocelot->fwd_domain_lock.

In the interest of simple locking rules, make all MAC Merge layer state
changes (and preemptible traffic class changes) be serialized by the
ocelot->fwd_domain_lock.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>