[$] Resistance to Rust abstractions for DMA mapping

While the path toward the ability to write device drivers in Rust has beenanything but smooth, steady progress has been made and that goal is closeto being achieved - for some types of drivers at least. Device driversneed to be able to set up memory areas for direct memory access (DMA)transfers, though; that means Rust drivers will need a set ofabstractions to interface with the kernel's DMA-mapping subsystem. Thoseabstractions have run into resistance that has the potential to blockprogress on the Rust-for-Linux project as a whole.