mptcp: clear scheduled subflows on retransmit

When __mptcp_retrans() kicks-in, it schedules one or more subflows for
retransmission, but such subflows could be actually left alone if there
is no more data to retransmit and/or in case of concurrent fallback.

Scheduled subflows could be processed much later in time, i.e. when new
data will be transmitted, leading to bad subflow selection.

Explicitly clear all scheduled subflows before leaving the
retransmission function.

Fixes: ee2708aedad0 ("mptcp: use get_retrans wrapper")
Cc: [e-mail]
Reported-by: Filip Pokryvka <[e-mail]>
Signed-off-by: Paolo Abeni <[e-mail]>
Reviewed-by: Matthieu Baerts (NGI0) <[e-mail]>
Signed-off-by: Matthieu Baerts (NGI0) <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/mptcp/protocol.c

···
 		}
 
 		if (!mptcp_send_head(sk))
-			return;
+			goto clear_scheduled;
 
 		goto reset_timer;
 	}
···
 			if (__mptcp_check_fallback(msk)) {
 				spin_unlock_bh(&msk->fallback_lock);
 				release_sock(ssk);
-				return;
+				goto clear_scheduled;
 			}
 
 			while (info.sent < info.limit) {
···
 
 	if (!mptcp_rtx_timer_pending(sk))
 		mptcp_reset_rtx_timer(sk);
+
+clear_scheduled:
+	/* If no rtx data was available or in case of fallback, there
+	 * could be left-over scheduled subflows; clear them all
+	 * or later xmit could use bad ones
+	 */
+	mptcp_for_each_subflow(msk, subflow)
+		if (READ_ONCE(subflow->scheduled))
+			mptcp_subflow_set_scheduled(subflow, false);
 }
 
 /* schedule the timeout timer for the relevant event: either close timeout