btrfs: rename btrfs_bio::iter field

Rename iter to saved_iter and move it next to the repair internals
and nothing outside of bio.c should be touching it.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>