RAS/CEC: Convert the timer callback to a workqueue

cec_timer_fn() is a timer callback which reads ce_arr.array[] and
updates its decay values. However, it runs in interrupt context and the
mutex protection the CEC uses for that array, is inadequate. Convert the
used timer to a workqueue to keep the tasks the CEC performs preemptible
and thus low-prio.

 [ bp: Rewrite commit message.
   s/timer/decay/gi to make it agnostic as to what facility is used. ]

Fixes: 011d8261 ("RAS: Add a Corrected Errors Collector")
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20190416213351.28999-2-xiyou.wangcong@gmail.com