blkio-throttle: There is no need to convert jiffies to milli seconds

o Do not convert jiffies to mili seconds as it is not required. Just work
  with jiffies and HZ.

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