Revert "mptcp: Use correct MSS for space-computation"

This reverts commit 1ddc334.

It has been reported that the commit introducing the issue fixed by this
patch is causing issues.

Best to revert this if it is causing more issues than what it solves on
these old kernels.

Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>

net/mptcp/mptcp_sched.c

@@ -70,19 +70,19 @@ static bool mptcp_is_temp_unavailable(struct sock *sk,
 	if (in_flight >= tp->snd_cwnd)
 		return true;
 
-	mss_now = tcp_current_mss(sk);
-
 	/* Now, check if what is queued in the subflow's send-queue
 	 * already fills the cwnd.
 	 */
-	space = (tp->snd_cwnd - in_flight) * mss_now;
+	space = (tp->snd_cwnd - in_flight) * tp->mss_cache;
 
 	if (tp->write_seq - tp->snd_nxt >= space)
 		return true;
 
 	if (zero_wnd_test && !before(tp->write_seq, tcp_wnd_end(tp)))
 		return true;
 
+	mss_now = tcp_current_mss(sk);
+
 	/* Don't send on this subflow if we bypass the allowed send-window at
 	 * the per-subflow level. Similar to tcp_snd_wnd_test, but manually
 	 * calculated end_seq (because here at this point end_seq is still at