geneve: check MTU for a minimum in geneve_change_mtu()

geneve_change_mtu() will be used not only as ndo_change_mtu() callback,
but also to verify a user specified MTU on a new link creation in the
next patch.

Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/geneve.c

@@ -942,11 +942,10 @@ tx_error:
 
 static int geneve_change_mtu(struct net_device *dev, int new_mtu)
 {
-	/* Only possible if called internally, ndo_change_mtu path's new_mtu
-	 * is guaranteed to be between dev->min_mtu and dev->max_mtu.
-	 */
 	if (new_mtu > dev->max_mtu)
 		new_mtu = dev->max_mtu;
+	else if (new_mtu < dev->min_mtu)
+		new_mtu = dev->min_mtu;
 
 	dev->mtu = new_mtu;
 	return 0;