btrfs: send: remove unused type parameter to iterate_inode_ref_t

Again, I don't think this was ever used since iterate_dir_item() is only
used for xattrs. No functional change.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>