[$] Rust and C filesystem APIs

As the Rust-for-Linux projectadvances, the kernel is gradually accumulating abstraction layers that enable Rust code to interface with theexisting C code. As the discussion around the set of filesystemabstractions posted by Wedson Almeida Filho in December shows, though,there is some tension between two approaches to the design of thoseabstractions. The approach favored by most of the kernel's C programmerslooks set to win out, but this is a discussion that is likely to return asthe use of Rust in the kernel grows.