io_uring: don't leak creds on SQO attach error

Attaching to already dead/dying SQPOLL task is disallowed in
io_sq_offload_create(), but cleanup is hand coded by calling
io_put_sq_data()/etc., that miss to put ctx->sq_creds.

Defer everything to error-path io_sq_thread_finish(), adding
ctx->sqd_list in the error case as well as finish will handle it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>