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>