UPSTREAM: mptcp: give rcvlowat some love

The MPTCP protocol allow setting sk_rcvlowat, but the value there
is currently ignored.

Additionally, the default subflows sk_rcvlowat basically disables per
subflow delayed ack: the MPTCP protocol move the incoming data from the
subflows into the msk socket as soon as the TCP stacks invokes the subflow
data_ready callback. Later, when __tcp_ack_snd_check() takes action,
the subflow-level copied_seq matches rcv_nxt, and that mandate for an
immediate ack.

Let the mptcp receive path be aware of such threshold, explicitly tracking
the amount of data available to be ready and checking vs sk_rcvlowat in
mptcp_poll() and before waking-up readers.

Additionally implement the set_rcvlowat() callback, to properly handle
the rcvbuf auto-tuning on sk_rcvlowat changes.

Finally to properly handle delayed ack, force the subflow level threshold
to 0 and instead explicitly ask for an immediate ack when the msk level th
is not reached.

Bug: 356178496
Change-Id: I6f8c952e8350db1061da1f7379e4431b121073e2
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231023-send-net-next-20231023-2-v1-5-9dc60939d371@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit 5684ab1a)
Signed-off-by: Subash Abhinov Kasiviswanathan <quic_subashab@quicinc.com>