aoe: clean device rq_list in aoedev_downdev()

In the Linux kernel, the following vulnerability has been resolved:

aoe: clean device rq_list in aoedev_downdev()

An aoe device's rq_list contains accepted block requests that are

waiting to be transmitted to the aoe target. This queue was added as

part of the conversion to blk_mq. However, the queue was not cleaned out

when an aoe device is downed which caused blk_mq_freeze_queue() to sleep

indefinitely waiting for those requests to complete, causing a hang. This

fix cleans out the queue before calling blk_mq_freeze_queue().