raid6check: get device ordering correct for syndrome calculation.

raid6check: get device ordering correct for syndrome calculation.

The order of devices used for the syndrome calculation is not
the same as the order of data in the array.
The D block immediately after Q is first, then they continue
cyclicly in raid-disk order, skipping over the P disk if it is seen.

This gets the 'check' right for all layouts other than DDF, which is
quite different.

I haven't confirmed that this does't break repair.

Signed-off-by: NeilBrown <neilb@suse.de>