bonding: fix link down handling in 802.3ad mode

One of the purposes of bonding is to allow for redundant links, and failover
correctly if the cable is pulled. If all the members of a bonded device have
no carrier present, the bonded device itself needs to report no carrier present
to user space so management tools (like routing daemons) can respond.

Bonding in 802.3ad mode does not work correctly for this because it incorrectly
chooses a link that is down as a possible aggregator.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>