[$] A generic ring buffer for the kernel

The kernel's user-space ABI does not lack for ring buffers; they have been
defined for subsystems like BPF, io_uring, perf,
and tracing, for
example. Naturally, each of those ring buffers is unique, with no common
interface between them. The natural response to this ABI proliferation is,
of course, to add yet another ring buffer as the generic option; that is
the intent of this
patch series from Kent Overstreet adding a new set of system calls for
ring buffers.