dsched_fq - Avoid int64 overflow on total_budget

dsched_fq - Avoid int64 overflow on total_budget

* Avoid an int64 overflow when calculating the total disk budget by
  losing bits of precision if needed.

* Note that this might not quite fix the issue yet, as there is one
  other place where the int64 overflow can happen, although it is less
  likely.

* While here, make the rebalancing happen every 0.5s instead of every
  1s, effectively reducing the chance of int64 overflows.

Reported-by: Antonio Huete (tuxillo@)