sit: replace 68 with micro IPV4_MIN_MTU

Use meaningfull micro IPV4_MIN_MTU
Signed-off-by: zhang kai <zhangkaiheb@126.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -970,7 +970,7 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 	if (df) {
 		mtu = dst_mtu(&rt->dst) - t_hlen;
 
-		if (mtu < 68) {
+		if (mtu < IPV4_MIN_MTU) {
 			dev->stats.collisions++;
 			ip_rt_put(rt);
 			goto tx_error;