net: call call_netdevice_unregister_net_notifiers from unregister

The function does the same thing as the existing code, so rather call
call_netdevice_unregister_net_notifiers() instead of code duplication.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>