That's exactly the point. The patch in question is not to be used in any part of the kernel. It is a wrapper around the C API. It has no impact on current code and it will only be used by Rust drivers using DMA, the people of Rust for Linux were clear about it.
Duplicating this wrapper in every driver would just make painful the usage of DMA for Rust drivers for not benefit.
The concept of safety is pretty different in a kernel and in a programming language. The kernel can be attacked directly with malicious calls, while it's the program created by the rust compiler that can be attacked (unless you envision Rust as a sandboxing technology, what it is absolutely not).
The safety in a programming language is mostly protecting the programmer against itself. The probability for a programmer to write this kind of code by mistake is close to zero, as opposed to UB in C or C++ that are pretty common. To make a vulnerable program with this kind of issue, the programmer would have to make them on purpose, what is unlikely unless for this kind of joke repository.
Duplicating this wrapper in every driver would just make painful the usage of DMA for Rust drivers for not benefit.