chan_dahdi: PRI span status may stay "Down, Active" after a short alarm

Upon a short PRI disconnection, libpri may maintain Q.921 layer 'up' and
may thus not send PRI_EVENT_DCHAN_DOWN / PRI_EVENT_DCHAN_UP events.
If pri_event_alarm() clears DCHAN_UP status bit upon alarm detection
and no Q.921 reconnection sequence occurs, chan_dahdi will keep
seeing span status "Down" at the end of alarm.

This patch modifies pri_event_alarm() in order to keep DCHAN_UP bit
unchanged. libpri will send a PRI_EVENT_DCHAN_DOWN event if it detects
a disconnection of Q.921 layer and this will clear DCHAN_UP if required.

ASTERISK-28615

Change-Id: Ibe27df4971fd4c82cc6850020bce4a8b2692c996

channels/sig_pri.c

@@ -2243,7 +2243,7 @@ static void *pri_ss_thread(void *data)
 
 void pri_event_alarm(struct sig_pri_span *pri, int index, int before_start_pri)
 {
-	pri->dchanavail[index] &= ~(DCHAN_NOTINALARM | DCHAN_UP);
+	pri->dchanavail[index] &= ~DCHAN_NOTINALARM;
 	if (!before_start_pri) {
 		pri_find_dchan(pri);
 	}