io_uring/net: improve io_get_notif_slot types

Don't use signed int for slot indexing.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/e4d15aefebb5e55729dd9b5ec01ab16b70033343.1658742118.git.asml.silence@gmail.com


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