liburing: add api to set napi busy poll settings

This adds two functions to manage the napi busy poll settings:
- io_uring_register_napi
- io_uring_unregister_napi

Signed-off-by: Stefan Roesch <shr@devkernel.io>
Reviewed-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Link: https://lore.kernel.org/r/20230425182054.2826621-2-shr@devkernel.io
[axboe: fix nr_args == 0 for register call, as per Olivier Langlois]
Signed-off-by: Jens Axboe <axboe@kernel.dk>