Fix kern/39769: race condition in TCP timers
When a TCP timer is disarmed (with callout_stop()) in the general case
callout_invoking() isn't checked, so the timer handler could be called run
when the current interrupt handler exits, athough the timer is disarmed.
This case cause bad things like TCPT_REXMT and TCPT_PERSIST being both pending,
causing a panic (see the PR for details).
Close the issue by aborting the handler if the timer is not callout_expired().
(the EXPIRED flag being cleared by callout_stop()).

(bouyer)