blk-mq: dont assume rq->errors is set when returning an error from ->queue_rq

rq->errors never has been part of the communication protocol between drivers
and the block stack and most drivers will not have initialized it.

Return -EIO to upper layers when the driver returns BLK_MQ_RQ_QUEUE_ERROR
unconditionally.  If a driver want to return a different error it can easily
do so by returning success after calling blk_mq_end_io itself.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>