Currently, we need to go through raw pointers and then re-create the
`NonNull` from the result of offsetting the raw pointer.

`feature(non_null_convenience)` [1] has been stabilized in Rust
1.80.0 [2], which is older than our new minimum Rust version
(Rust 1.85.0).

Thus, now that we bump the Rust minimum version, simplify using
`NonNull::add()` and clean the TODO note.

Link: https://github.com/rust-lang/rust/issues/117691 [1]
Link: https://github.com/rust-lang/rust/pull/124498 [2]
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260405235309.418950-15-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Introduce the VmallocPageIter type; an instance of VmallocPageIter may
be exposed by owners of vmalloc allocations to provide borrowed access
to its backing pages.

For instance, this is useful to access and borrow the backing pages of
allocation primitives, such as Box and Vec, backing a scatterlist.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250820145434.94745-4-dakr@kernel.org
[ Drop VmallocPageIter::base_address(), move to allocator/iter.rs and
  stub VmallocPageIter for allocator_test.rs. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>