[PATCH RESEND] Fix and extend raid6check repair

Hello,

this is a resend of my patches dated Jul 20th in the "Find mismatch in 
data blocks during raid6 repair" thread.

Currently, the raid6check tool does not compile (due to the xmalloc 
changes) and my initial set of repair patches are faulty.  Please merge 
and see my original emails for extended comments
http://thread.gmane.org/gmane.linux.raid/38922/focus=39454
http://thread.gmane.org/gmane.linux.raid/38922/focus=39460



Cheers

Robert