bonding: properly unset current_arp_slave on slave link up

When a slave comes up, we're unsetting the current_arp_slave without
removing active flags from it, which can lead to situations where we have
more than one slave with active flags in active-backup mode.

To avoid this situation we must remove the active flags from a slave before
removing it as a current_arp_slave.

Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Marcelo Ricardo Leitner <mleitner@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>