io_uring/bpf-ops: implement loop_step with BPF struct_ops

io_uring/bpf-ops: implement loop_step with BPF struct_ops

Introduce io_uring BPF struct ops implementing the loop_step callback,
which will allow BPF to overwrite the default io_uring event loop logic.

The callback takes an io_uring context, the main role of which is to be
passed to io_uring kfuncs. The other argument is a struct iou_loop_params,
which BPF can use to request CQ waiting and communicate other parameters.
See the event loop description in the previous patch for more details.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://patch.msgid.link/98db437651ce64e9cbeb611c60bf5887259db09f.1772109579.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>