tcp: Allow exceeding the available sending buffer size in window advertisements

If the remote peer is advertising a bigger value than our current
sending buffer, it means that a bigger sending buffer is likely to
benefit throughput.

We can get a bigger sending buffer by means of the buffer size
auto-tuning performed by the Linux kernel, which is triggered by
aggressively filling the sending buffer.

Use an adaptive boost factor, up to 150%, depending on:

- how much data we sent so far: we don't want to risk retransmissions
  for short-lived connections, as the latency cost would be
  unacceptable, and

- the current RTT value, as we need a bigger buffer for higher
  transmission delays

The factor we use is not quite a bandwidth-delay product, as we're
missing the time component of the bandwidth, which is not interesting
here: we are trying to make the buffer grow at the beginning of a
connection, progressively, as more data is sent.

The tuning of the amount of boost factor we want to apply was done
somewhat empirically but it appears to yield the available throughput
in rather different scenarios (from ~ 10 Gbps bandwidth with 500ns to
~ 1 Gbps with 300 ms RTT) and it allows getting there rather quickly,
within a few seconds for the 300 ms case.

Note that we want to apply this factor only if the window advertised
by the peer is bigger than the current sending buffer, as we only need
this for auto-tuning, and we absolutely don't want to incur
unnecessary retransmissions otherwise.

The related condition in tcp_update_seqack_wnd() is not redundant as
there's a subtractive factor, sendq, in the calculation of the window
limit. If the sending buffer is smaller than the peer's advertised
window, the additional limit we might apply might be lower than we
would do otherwise.

Assuming that the sending buffer is reported as 100k, sendq is
20k, we could have these example cases:

1. tinfo->tcpi_snd_wnd is 120k, which is bigger than the sending
   buffer, so we boost its size to 150k, and we limit the window
   to 120k

2. tinfo->tcpi_snd_wnd is 90k, which is smaller than the sending
   buffer, so we aren't trying to trigger buffer auto-tuning and
   we'll stick to the existing, more conservative calculation,
   by limiting the window to 100 - 20 = 80k

If we omitted the new condition, we would always use the boosted
value, that is, 120k, even if potentially causing unnecessary
retransmissions.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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