raid5.c internal stripe layout

Hi,

I'm wondering if there are any pros/cons for handling stripes in memory
exactly in the same layout as on disks. There are several places that could
be simplified if parity disks would be always at the end (DDD..DDPQ). Of 
course one would have to do the mapping during read/write operations.

Cases for optimization would be:

- replace conditions "i == pd_idx || i == qd_qdx" with "i>=pd_idx"
- run data disk loops starting/ending with pd_idx (triple parity raid someday?)
- remove syndrome disk order calculation functions
- Shaohua's last patch could cross chunks
- ...

Just in case I missed a discussion I'm interested if patches are welcome.

Markus