igb: Update MTU so that it is always at least a standard frame size

This change makes it so that we limit the lower bound for max_frame_size to
the size of a standard Ethernet frame.  This allows for feature parity with
other Intel based drivers such as ixgbe.
Signed-off-by: NAlexander Duyck <alexander.h.duyck@intel.com>
Tested-by: NAaron Brown <aaron.f.brown@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -4813,6 +4813,10 @@ static int igb_change_mtu(struct net_device *netdev, int new_mtu)
 		return -EINVAL;
 	}
 
+	/* adjust max frame to be at least the size of a standard frame */
+	if (max_frame < (ETH_FRAME_LEN + ETH_FCS_LEN))
+		max_frame = ETH_FRAME_LEN + ETH_FCS_LEN;
+
 	while (test_and_set_bit(__IGB_RESETTING, &adapter->state))
 		msleep(1);