igb: limit minimum mtu to 68 to keep ip bound to interface

Limit the minimum mtu to 68 in order to prevent ip from being unbound from
the interface.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/igb/igb_main.c

@@ -3709,8 +3709,7 @@ static int igb_change_mtu(struct net_device *netdev, int new_mtu)
 	int max_frame = new_mtu + ETH_HLEN + ETH_FCS_LEN;
 	u32 rx_buffer_len, i;
 
-	if ((max_frame < ETH_ZLEN + ETH_FCS_LEN) ||
-	    (max_frame > MAX_JUMBO_FRAME_SIZE)) {
+	if ((new_mtu < 68) || (max_frame > MAX_JUMBO_FRAME_SIZE)) {
 		dev_err(&pdev->dev, "Invalid MTU setting\n");
 		return -EINVAL;
 	}