io_uring: use the right type for creds iteration

io_uring: use the right type for creds iteration

In io_ring_ctx_wait_and_kill(), struct creds *creds is used to
iterate and prune credentials. But the correct type is struct cred.
This doesn't matter as the variable isn't used at all, only the index
is used. But it's confusing using a type that isn't valid, so fix it
up.

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