block, blk-sysfs: Use the variable directly instead of a function call

In the function blk_register_queue(), var _dev_ is already assigned by
disk_to_dev().So use it directly instead of calling disk_to_dev() again.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Modified by me to delete an empty line in the same function while
in there anyway.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>