This is a follow on to LU-13814, and so it won't be landable until all of that code has landed.

Once cl_page usage for DIO has been replaced with the OSC transfer page, there are further improvements available in memory usage and efficiency.

OSC transfer pages are still placed on lists to go in BRW RPCs, but this is not required for DIO and can be replaced with an array.  This allows the removal of several lists and significant reductions in the size of the OSC transfer page.  This should provide significant performance boosts beyond those in LU-13814, allowing us to hit >100 GiB/s from one thread doing DIO.