Looking at the patch
(http://git.kernel.dk/?p=linux-2.6-block.git;a=commitdiff;h=761e433f3de6fb8e369af9e5c08beb86286d023f)

I'm not sure it's an optimal design.  The flush callback really
is a per-queue thing.  Why isn't it a function pointer in the request
queue when doing the blk_run_queue call once we're done with a given
queue before moving on to the next one?