super: use common iterator (Part 2)

Use a common iterator for all callbacks. We could go for something even
more elaborate (advance step-by-step similar to iov_iter) but I really
don't think this is warranted.

Link: https://lore.kernel.org/r/20250329-work-freeze-v2-5-a47af37ecc3d@kernel.org


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>