io_uring/kbuf: remove legacy kbuf caching

Remove all struct io_buffer caches. It makes it a fair bit simpler.
Apart from from killing a bunch of lines and juggling between lists,
__io_put_kbuf_list() doesn't need ->completion_lock locking now.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/18287217466ee2576ea0b1e72daccf7b22c7e856.1738724373.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>