OVPN_HEAD_ROOM
if (unlikely(skb_cow_head(skb, OVPN_HEAD_ROOM)))
dev->mtu = ETH_DATA_LEN - OVPN_HEAD_ROOM;
dev->max_mtu = IP_MAX_MTU - OVPN_HEAD_ROOM;
dev->needed_headroom = ALIGN(OVPN_HEAD_ROOM, 4);