io_uring/zcrx: allow synchronous buffer return

io_uring/zcrx: allow synchronous buffer return

Returning buffers via a ring is performant and convenient, but it
becomes a problem when/if the user misconfigured the ring size and it
becomes full. Add a synchronous way to return buffers back to the page
pool via a new register opcode. It's supposed to be a reliable slow
path for refilling.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>