dsa_user_change_mtu
dsa_user_change_mtu(user, user->mtu);
.ndo_change_mtu = dsa_user_change_mtu,
ret = dsa_user_change_mtu(user_dev, ETH_DATA_LEN);
err = dsa_user_change_mtu(dev, dev->mtu);
int dsa_user_change_mtu(struct net_device *dev, int new_mtu);