[TCP]: use non-delayed ACK for congestion control RTT

When a delayed ACK representing two packets arrives, there are two RTT
samples available, one for each packet.  The first (in order of seq
number) will be artificially long due to the delay waiting for the
second packet, the second will trigger the ACK and so will not itself
be delayed.

According to rfc1323, the SRTT used for RTO calculation should use the
first rtt, so receivers echo the timestamp from the first packet in
the delayed ack.  For congestion control however, it seems measuring
delayed ack delay is not desirable as it varies independently of
congestion.

The patch below causes seq_rtt and last_ackt to be updated with any
available later packet rtts which should have less (and hopefully
zero) delack delay.  The rtt value then gets passed to
ca_ops->pkts_acked().

Where TCP_CONG_RTT_STAMP was set, effort was made to supress RTTs from
within a TSO chunk (!fully_acked), using only the final ACK (which
includes any TSO delay) to generate RTTs.  This patch removes these
checks so RTTs are passed for each ACK to ca_ops->pkts_acked().

For non-delay based congestion control (cubic, h-tcp), rtt is
sometimes used for rtt-scaling.  In shortening the RTT, this may make
them a little less aggressive.  Delay-based schemes (eg vegas, veno,
illinois) should get a cleaner, more accurate congestion signal,
particularly for small cwnds.  The congestion control module can
potentially also filter out bad RTTs due to the delayed ack alarm by
looking at the associated cnt which (where delayed acking is in use)
should probably be 1 if the alarm went off or greater if the ACK was
triggered by a packet.

Signed-off-by: Gavin McCullagh <gavin.mccullagh@nuim.ie>
Acked-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>