bonding: improve nl error msg when device can't be enslaved because of IFF_MASTER

Use a more user friendly netlink error message when a device can't be
enslaved because it has IFF_MASTER, by not referring directly to a
kernel internal flag.
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -1759,7 +1759,7 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
 	if (slave_dev->flags & IFF_MASTER &&
 	    !netif_is_bond_master(slave_dev)) {
 		BOND_NL_ERR(bond_dev, extack,
-			    "Device with IFF_MASTER cannot be enslaved");
+			    "Device type (master device) cannot be enslaved");
 		return -EPERM;
 	}