Merge branch 'tcp-new-mechanism-to-ACK-immediately'

Yuchung Cheng says:

====================
tcp: new mechanism to ACK immediately

This patch is a follow-up feature improvement to the recent fixes on
the performance issues in ECN (delayed) ACKs. Many of the fixes use
tcp_enter_quickack_mode routine to force immediate ACKs. However the
routine also reset tracking interactive session. This is not ideal
because these immediate ACKs are required by protocol specifics
unrelated to the interactiveness nature of the application.

This patch set introduces a new flag to send a one-time immediate ACK
without changing the status of interactive session tracking. With this
patch set the immediate ACKs are generated upon these protocol states:

1) When a hole is repaired
2) When CE status changes between subsequent data packets received
3) When a data packet carries CWR flag
====================

Signed-off-by: David S. Miller <davem@davemloft.net>