Reapply "loop: Add sanity check for read/write_iter"

This reverts commit c979a699.

It is now fixed upstream, so this can be reapplied.

Bug: 417466278
Bug: 417627880
Cc: Tudor Ambarus <tudordana@google.com>
Cc: Lucas Henneman <henneman@google.com>
Cc: Carlos Llamas <cmllamas@google.com>
Change-Id: I3894b4ab203b6eafcb19d4149729809e079e76bf
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>