mlxsw: spectrum_router: IPv6 events: Use tracker helpers to hold & put netdevices

Using the tracking helpers makes it easier to debug netdevice refcount
imbalances when CONFIG_NET_DEV_REFCNT_TRACKER is enabled.

Convert dev_hold() / dev_put() to netdev_hold() / netdev_put() in the
router code that deals with IPv6 address events.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/f0af6ad4722b4ca6e598fd4fda8311a3041651ec.1690471775.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>