rust: alloc: kbox: implement AsPageIter for VBox

Implement AsPageIter for VBox; this allows to iterate and borrow the
backing pages of a VBox. This, for instance, is useful in combination
with VBox backing a scatterlist.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250820145434.94745-6-dakr@kernel.org


Signed-off-by: Danilo Krummrich <dakr@kernel.org>