We somehow never free the idr, even though we init it for every ctx.
Free it when the rest of the ring data is freed.

Fixes: 071698e1 ("io_uring: allow registering credentials")
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>