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: 6f8a612a33e4 ("mptcp: keep track of advertised windows right edge")
Cc: [e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>
Reviewed-by: Mat Martineau <[e-mail]>
Signed-off-by: Matthieu Baerts (NGI0) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/mptcp/protocol.c

···
 	msk->write_seq = subflow_req->idsn + 1;
 	msk->snd_nxt = msk->write_seq;
 	msk->snd_una = msk->write_seq;
-	msk->wnd_end = msk->snd_nxt + req->rsk_rcv_wnd;
+	msk->wnd_end = msk->snd_nxt + tcp_sk(ssk)->snd_wnd;
 	msk->setsockopt_seq = mptcp_sk(sk)->setsockopt_seq;
 	mptcp_init_sched(msk, mptcp_sk(sk)->sched);