md: fix misapplied patch in raid5.c

commit 4ae3f847 ("md: raid5: fix
clearing of biofill operations") did not get applied correctly,
presumably due to substantial similarities between handle_stripe5 and
handle_stripe6.

This patch moves the chunk of new code from handle_stripe6 (where it isn't
needed (yet)) to handle_stripe5.

Signed-off-by: Neil Brown <neilb@suse.de>
Cc: "Dan Williams" <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>