io_uring: allow finding next link independent of req reference count

We currently try and start the next link when we put the request, and
only if we were going to free it. This means that the optimization to
continue executing requests from the same context often fails, as we're
not putting the final reference.

Add REQ_F_LINK_NEXT to keep track of this, and allow io_uring to find the
next request more efficiently.

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