writeback: I believe these defines were calculated with
 HZ=1000 in mind. Since our devices are HZ=100 I hardcoded the values on this
 constants. I'm not 100% sure if this code was made for HZ with 1000 but I
 couldn't track it to understand clearly. Although I didn't see any drawback
 of using this as it is now. If anyone knows the answer please chime in.

Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>