mlx5: register lag notifier for init network namespace only

The current code causes problems when the unregistering netdevice could
be different then the registering one.

Since the check in mlx5_lag_netdev_event() does not allow any other
network namespace anyway, fix this by registerting the lag notifier
per init network namespace only.

Fixes: d48834f9 ("mlx5: Use dev_net netdevice notifier registrations")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Tested-by: Aya Levin <ayal@mellanox.com>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>