mlx5: Add missing init_net check in FIB notifier

Take only FIB events that are happening in init_net into account. No other
namespaces are supported.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>