From: NeilBrown <neilb@cse.unsw.edu.au>

This condtion on this loop is primarily to avoid the loop if it doesn't
appear to be needed.  However it optimises a little too much and there is a
case where it skips the loop when it is really needed.  This patch fixes
it.

This is the raid6 version of the recent raid5 resync bugfix.
Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>