mptcp: pm: ADD_ADDR rtx: resched blocked ADD_ADDR quicker

When an ADD_ADDR needs to be retransmitted and another one has already
been prepared -- e.g. multiple ADD_ADDRs have been sent in a row and
need to be retransmitted later -- this additional retransmission will
need to wait.

In this case, the timer was reset to TCP_RTO_MAX / 8, which is ~15
seconds. This delay is unnecessary long: it should just be rescheduled
at the next opportunity, e.g. after the retransmission timeout.

Without this modification, some issues can be seen from time to time in
the selftests when multiple ADD_ADDRs are sent, and the host takes time
to process them, e.g. the "signal addresses, ADD_ADDR timeout" MPTCP
Join selftest, especially with a debug kernel config.

Note that on older kernels, 'timeout' is not available. It should be
enough to replace it by one second (HZ).

Fixes: 00cfd77 ("mptcp: retransmit ADD_ADDR when timeout")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260505-net-mptcp-pm-fixes-7-1-rc3-v1-6-fca8091060a4@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: matttbe

net/mptcp/pm.c

@@ -342,13 +342,8 @@ static void mptcp_pm_add_timer(struct timer_list *timer)
 		goto out;
 	}
 
-	if (mptcp_pm_should_add_signal_addr(msk)) {
-		timeout = TCP_RTO_MAX / 8;
-		goto out;
-	}
-
 	timeout = mptcp_adjust_add_addr_timeout(msk);
-	if (!timeout)
+	if (!timeout || mptcp_pm_should_add_signal_addr(msk))
 		goto out;
 
 	spin_lock_bh(&msk->pm.lock);