tcp: reduce the bloat caused by tcp_is_cwnd_limited()
tcp_is_cwnd_limited() allows GSO/TSO enabled flows to increase
their cwnd to allow a full size (64KB) TSO packet to be sent.
Non GSO flows only allow an extra room of 3 MSS.
For most flows with a BDP below 10 MSS, this results in a bloat
of cwnd reaching 90, and an inflate of RTT.
Thanks to TSO auto sizing, we can restrict the bloat to the number
of MSS contained in a TSO packet (tp->xmit_size_goal_segs), to keep
original intent without performance impact.
Because we keep cwnd small, it helps to keep TSO packet size to their
optimal value.
Example for a 10Mbit flow, with low TCP Small queue limits (no more than
2 skb in qdisc/device tx ring)