mptcp: fix snd_wnd initialization for passive socket

Such value should be inherited from the first subflow, but
passive sockets always used 'rsk_rcv_wnd'.

Fixes: 6f8a612a

 ("mptcp: keep track of advertised windows right edge")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240223-upstream-net-20240223-misc-fixes-v1-5-162e87e48497@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>