block/genhd: Change some numerals into macros

Rename the numerals in the diskstats_show() into the macros.

Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>