bonding: in balance-rr mode, set curr_active_slave only if it is up

If all slaves of a balance-rr bond with ARP monitor are enslaved
with down link state, bond keeps down state even after slaves
go up.

This is caused by bond_enslave() setting curr_active_slave to
first slave not taking into account its link state. As
bond_loadbalance_arp_mon() uses curr_active_slave to identify
whether slave's down->up transition should update bond's link
state, bond stays down even if slaves are up (until first slave
goes from up to down at least once).

Before commit f31c7937 "bonding: start slaves with link down for
ARP monitor", this was masked by slaves always starting in UP
state with ARP monitor (and MII monitor not relying on
curr_active_slave being NULL if there is no slave up).
Signed-off-by: NMichal Kubecek <mkubecek@suse.cz>
Signed-off-by: NJay Vosburgh <fubar@us.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -1838,7 +1838,7 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev)
 		 * anyway (it holds no special properties of the bond device),
 		 * so we can change it without calling change_active_interface()
 		 */
-		if (!bond->curr_active_slave)
+		if (!bond->curr_active_slave && new_slave->link == BOND_LINK_UP)
 			bond->curr_active_slave = new_slave;
 
 		break;