blkio: Do not export throttle files if CONFIG_BLK_DEV_THROTTLING=n

Currently throttling related files were visible even if user had disabled
throttling using config options. It was switching off background throttling
of bio but not the cgroup files. This patch fixes it.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>