blk-sysfs: add a new attr_group for blk_mq

Currently wbt sysfs entry is created for bio based device, and wbt can
be enabled for such device through sysfs while it doesn't make sense
because wbt can only work for rq based device. In the meantime, there
are other similar sysfs entries.

Fix this by adding a new attr_group for blk_mq, and sysfs entries will
only be created when the device is rq based.

Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230527010644.647900-6-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>