net: dsa: mv88e6xxx: fix use-after-free in mv88e6xxx_mdios_unregister

net: dsa: mv88e6xxx: fix use-after-free in mv88e6xxx_mdios_unregister

Since struct mv88e6xxx_mdio_bus *mdio_bus is the bus->priv of something
allocated with mdiobus_alloc_size(), this means that mdiobus_free(bus)
will free the memory backing the mdio_bus as well. Therefore, the
mdio_bus->list element is freed memory, but we continue to iterate
through the list of MDIO buses using that list element.

To fix this, use the proper list iterator that handles element deletion
by keeping a copy of the list element next pointer.

Fixes: f53a2ce893b2 ("net: dsa: mv88e6xxx: don't use devres for mdiobus")
Reported-by: Rafael Richter <rafael.richter@gin.de>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20220210174017.3271099-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>