io_uring: poll: remove unnecessary req->ref set

We now don't need to set req->refcount for poll requests since the
reworked poll code ensures no request release race.
Signed-off-by: NHao Xu <howeyxu@tencent.com>
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/ec6fee45705890bdb968b0c175519242753c0215.1655371007.git.asml.silence@gmail.comSigned-off-by: NJens Axboe <axboe@kernel.dk>

io_uring/poll.c

@@ -689,7 +689,6 @@ int io_poll_add_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 	if ((flags & IORING_POLL_ADD_MULTI) && (req->flags & REQ_F_CQE_SKIP))
 		return -EINVAL;
 
-	io_req_set_refcount(req);
 	poll->events = io_poll_parse_events(sqe, flags);
 	return 0;
 }