io_uring: make timeout sequence == 0 mean no sequence

Currently we make sequence == 0 be the same as sequence == 1, but that's
not super useful if the intent is really to have a timeout that's just
a pure timeout.

If the user passes in sqe->off == 0, then don't apply any sequence logic
to the request, let it purely be driven by the timeout specified.

Reported-by: 李通洲 <carter.li@eoitek.com>
Reviewed-by: 李通洲 <carter.li@eoitek.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>