btrfs: stop looking at btrfs_bio->iter in index_one_bio

All the bios that index_one_bio operates on are the bios submitted by the
upper layer.  These are never resubmitted to an actual device by the
raid56 code, and thus the iter never changes from the initial state.
Thus we can always just use bi_iter directly as it will be the same as
the saved copy.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>