The MPTCP code uses the assumption that the tcp_win_from_space() helper
does not use any TCP-specific field, and thus works correctly operating
on an MPTCP socket.
The commit dfa2f0483360 ("tcp: get rid of sysctl_tcp_adv_win_scale") broke
such assumption, and as a consequence most MPTCP connections stall on
zero-window event due to auto-tuning changing the rcv buffer size quite
randomly.
Address the issue synching again the MPTCP auto-tuning code with the TCP
one. To achieve that, factor out the windows size logic in socket
independent helpers, and reuse them in mptcp_rcv_space_adjust(). The MPTCP
level scaling_ratio is selected as the minimum one from the all the subflows,
as a worst-case estimate.

Co-developed-by: Matthieu Baerts <[email protected]>
Signed-off-by: Matthieu Baerts <[email protected]>
Signed-off-by: Paolo Abeni <[email protected]>
Message-Id: <099977f9fe49d41ee0aa2eddefe5e2e09da3a3f1.1689836587.git.pabeni@redhat.com>