mptcp: rcvbuf auto-tuning improvement

mptcp: rcvbuf auto-tuning improvement

Apply to the MPTCP auto-tuning the same improvements introduced for the
TCP protocol by the merge commit 2da35e4b4df9 ("Merge branch
'tcp-receive-side-improvements'").

The main difference is that TCP subflow and the main MPTCP socket need
to account separately for OoO: MPTCP does not care for TCP-level OoO
and vice versa, as a consequence do not reflect MPTCP-level rcvbuf
increase due to OoO packets at the subflow level.

This refeactor additionally allow dropping the msk receive buffer update
at receive time, as the latter only intended to cope with subflow receive
buffer increase due to OoO packets.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/487
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/559
Reviewed-by: Geliang Tang <geliang@kernel.org>
Tested-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250927-net-next-mptcp-rcv-path-imp-v1-3-5da266aa9c1a@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>