rust: alloc: kvec: implement AsPageIter for VVec

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

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>