This was designed to be a really sterm throttling threshold: if dirty
memory reaches this level then perform writeback and actually wait on
it.

It doesn't work.  Because memory dirtiers are required to perform
writeback if the amount of dirty AND writeback memory exceeds
dirty_async_ratio.

So kill it, and rely just on the request queues being appropriately
scaled to the machine size (they are).

This is basically what 2.4 does.