Merge branch 'tcp-dsack-multi-seg'

Priyaranjan Jha says:

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tcp: improve handling of DSACK covering multiple segments

Currently, while processing DSACK, we assume DSACK covers only one
segment. This leads to significant underestimation of no. of duplicate
segments with LRO/GRO. Also, the existing SNMP counters, TCPDSACKRecv
and TCPDSACKOfoRecv, make similar assumption for DSACK, which makes them
unusable for estimating spurious retransmit rates.

This patch series fixes the segment accounting with DSACK, by estimating
number of duplicate segments based on: (DSACKed sequence range) / MSS.
It also introduces a new SNMP counter, TCPDSACKRecvSegs, which tracks
the estimated number of duplicate segments.
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Signed-off-by: David S. Miller <davem@davemloft.net>