[$] Rust and C filesystem APIs

As the Rust-for-Linux project
advances, the kernel is gradually
accumulating abstraction layers that enable Rust code to interface with the
existing C code. As the discussion around the set of filesystem
abstractions posted by Wedson Almeida Filho in December shows, though,
there is some tension between two approaches to the design of those
abstractions. The approach favored by most of the kernel's C programmers
looks set to win out, but this is a discussion that is likely to return as
the use of Rust in the kernel grows.