md/raid5: use bio_list for the list of bios to return.

This will make it easier to splice two lists together which will
be needed in future patch.

Signed-off-by: NeilBrown <neilb@suse.com>
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index d051442..1de82a6e 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -265,7 +265,7 @@
 	int dec_preread_active;
 	unsigned long ops_request;
 
-	struct bio *return_bi;
+	struct bio_list return_bi;
 	struct md_rdev *blocked_rdev;
 	int handle_bad_blocks;
 };