mptcp: keep rcv_mwnd_seq in sync with subflow rcv_wnd

mptcp: keep rcv_mwnd_seq in sync with subflow rcv_wnd

MPTCP shares a receive window across subflows and applies it at the
subflow level by adjusting each subflow's rcv_wnd when needed.  With
the new TCP tracking of the maximum advertised window sequence,
rcv_mwnd_seq must stay consistent with these subflow-level rcv_wnd
adjustments.

Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260309-tcp_rfc7323_retract_wnd_rfc-v3-2-4c7f96b1ec69@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>