rust: alloc: simplify with `NonNull::add()` now that it is stable

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>

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