void bond_change_active_slave (struct bonding * bond, struct slave * new_active);
Set the new slave to the bond's settings and unset them on the old curr_active_slave. Setting include flags, mc-list, promiscuity, allmulti, etc.
If new's link state is BOND_LINK_BACK we'll set it to BOND_LINK_UP, because it is apparently the best available slave we have, even though its updelay hasn't timed out yet.
Caller must hold curr_slave_lock for writing.