Merge branch 'tcp-more-reliable-window-probes'

Eric Dumazet says:

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tcp: more reliable window probes

This series address a problem caused by small rto_min timers in DC,
leading to either timer storms or early flow terminations.

We also add two new SNMP counters for proper monitoring :
TCPWinProbe and TCPKeepAlive

v2: added TCPKeepAlive counter, as suggested by Yuchung & Neal
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Signed-off-by: David S. Miller <davem@davemloft.net>