net: make dev->master general

dev->master is now tightly connected to bonding driver. This patch makes
this pointer more general and ready to be used by others.

 - netdev_set_master() - bond specifics moved to new function
   netdev_set_bond_master()
 - introduced netif_is_bond_slave() to check if device is a bonding slave

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/cxgb3/cxgb3_offload.c b/drivers/net/cxgb3/cxgb3_offload.c
index 7ea94b5..862804f 100644
--- a/drivers/net/cxgb3/cxgb3_offload.c
+++ b/drivers/net/cxgb3/cxgb3_offload.c
@@ -186,9 +186,10 @@
 				dev = NULL;
 				if (grp)
 					dev = vlan_group_get_device(grp, vlan);
-			} else
+			} else if (netif_is_bond_slave(dev)) {
 				while (dev->master)
 					dev = dev->master;
+			}
 			return dev;
 		}
 	}