block: add helper macros for queue sysfs entries

Add two helpers macros to avoid boilerplate code for the queue sysfs
entries.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>