[$] A generic ring buffer for the kernel

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