bonding: Change active slave quietly when bond is down

bond_change_active_slave() may be called when a slave is added, even
if the bond has not been brought up yet.  It may then attempt to send
packets, and further it may use mcast_work which is uninitialised
before the bond is brought up.  Add the necessary checks for
netif_running(bond->dev).

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>