TCP Cubic: After leaving slowstart fix unintended cwnd jump.
Initializing K to zero in D23655 introduced a miscalculation,
where cwnd would suddenly jump to cwnd_max instead of gradually
increasing, after leaving slow-start.
Properly calculating K instead of resetting it to zero resolves
this issue. Also making sure, that cwnd is recalculated at the
earliest opportunity once slow-start is over.