[PATCH] md: improve handing of read errors with raid6

This is a simple port of match functionality across from raid5.  If we get a
read error, we don't kick the drive straight away, but try to over-write with
good data first.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>