This adds an end_that_request_chunk() helper, that puts the core
functionality in __end_that_request_first().  This one deals in bytes.
end_that_request_first() does the 512 multiply, and then
end_that_request_chunk() can just use it directly.

This enables ide-cd to do proper dma in sizes that are not sector
aligned.  Some of the most important CD functions (ripping audio dma,
burning audio cds, burning raw cds) _require_ this or we will be in pio.
That stinks.  We simply cannot use pio for these, not on writers much
that are fast!