tcp: fix stretch ACK bugs in Yeah

Change Yeah to properly handle stretch ACKs in additive
increase mode by passing in the count of ACKed packets
to tcp_cong_avoid_ai().

In addition, we re-implemented the scalable path using
tcp_cong_avoid_ai() and removed the pkts_acked variable.

Signed-off-by: Pengcheng Yang <yangpc@wangsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>