bonding: warn user when 802.3ad speed is unknown

Goal is to advertise the user when ethtool speeds and 802.3ad speeds are
desynchronized.
When this case happens, the kernel needs to be patched.
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_3ad.c

@@ -322,6 +322,11 @@ static u16 __get_link_speed(struct port *port)
 
 		default:
 			/* unknown speed value from ethtool. shouldn't happen */
+			if (slave->speed != SPEED_UNKNOWN)
+				pr_warn_once("%s: unknown ethtool speed (%d) for port %d (set it to 0)\n",
+					     slave->bond->dev->name,
+					     slave->speed,
+					     port->actor_port_number);
 			speed = 0;
 			break;
 		}