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: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>