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>