One of the patches for LU-13799, "Implement lower/upper aio" (https://review.whamcloud.com/44209/) created a complicated setup where the cl_dio_aio struct was used both for the top level DIO or AIO and for the lower level sub I/Os (corresponding to stripes).

This is quite complicated and hard to follow, so this rewrites these two uses to be separate structs.  This incidentally fixes at least one possible memory leak.

This is an attempt to clean up the code to make some bugs easier to identify.  This may be a prelude to moving the top level AIO to a different struct from the DIO.