posix-timers: do_schedule_next_timer: fix the setting of ->si_overrun

do_schedule_next_timer() sets info->si_overrun = timr->it_overrun_last,
this discards the already accumulated overruns.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Mark McLoughlin <markmc@redhat.com>
Cc: Oliver Pinter <oliver.pntr@gmail.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: stable@kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/posix-timers.c

@@ -289,7 +289,7 @@ void do_schedule_next_timer(struct siginfo *info)
 		else
 			schedule_next_timer(timr);
 
-		info->si_overrun = timr->it_overrun_last;
+		info->si_overrun += timr->it_overrun_last;
 	}
 
 	if (timr)