[$] Resistance to Rust abstractions for DMA mapping

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