mptcp: Avoid over-scheduling on a subflow

When the scheduled data exactly fits the available space, we should no
more add more data to the subflow. Thus, the if-condition must be >=
space.

Reported-by: Anh Vu Vu <vuanh.vu@ikt.uni-hannover.de>
Fixes: Zero-day bug
Signed-off-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
(cherry picked from commit ec365b3)
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
(cherry picked from commit 7b3db0c)
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>

net/mptcp/mptcp_sched.c

@@ -75,7 +75,7 @@ static bool mptcp_is_temp_unavailable(struct sock *sk,
 	 */
 	space = (tp->snd_cwnd - in_flight) * tp->mss_cache;
 
-	if (tp->write_seq - tp->snd_nxt > space)
+	if (tp->write_seq - tp->snd_nxt >= space)
 		return true;
 
 	if (zero_wnd_test && !before(tp->write_seq, tcp_wnd_end(tp)))