btrfs: stop using the btrfs_bio saved iter in index_rbio_pages

The bios added to ->bio_list are the original bios fed into
btrfs_map_bio, which are never advanced.  Just use the iter in the
bio itself.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/raid56.c

@@ -1134,9 +1134,6 @@ static void index_rbio_pages(struct btrfs_raid_bio *rbio)
 		stripe_offset = start - rbio->bioc->raid_map[0];
 		page_index = stripe_offset >> PAGE_SHIFT;
 
-		if (bio_flagged(bio, BIO_CLONED))
-			bio->bi_iter = btrfs_bio(bio)->iter;
-
 		bio_for_each_segment(bvec, bio, iter) {
 			rbio->bio_pages[page_index + i] = bvec.bv_page;
 			i++;