mlxsw: spectrum_router: FIB: 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 FIB 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/5221a92e751c40447c55959f622267ccc999ed04.1690471774.git.petrm@nvidia.com


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