cfq-iosched: Convert from jiffies to nanoseconds

Convert all time-keeping in CFQ IO scheduler from jiffies to nanoseconds
so that we can later make the intervals more fine-grained than jiffies.
One jiffie is several miliseconds and even for today's rotating disks
that is a noticeable amount of time and thus we leave disk unnecessarily
idle.

Signed-off-by: Jeff Moyer <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>