io_uring: fix feeding io-wq with uninit reqs

io_steal_work() can't be sure that @nxt has req->work properly set, so we
can't pass it to io-wq as is.

A dirty quick fix -- drag it through io_req_task_queue(), and always
return NULL from io_steal_work().

e.g.

[   50.770161] BUG: kernel NULL pointer dereference, address: 00000000
[   50.770164] #PF: supervisor write access in kernel mode
[   50.770164] #PF: error_code(0x0002) - not-present page
[   50.770168] CPU: 1 PID: 1448 Comm: io_wqe_worker-0 Tainted: G
	I       5.8.0-rc2-00035-g2237d765-dirty #494
[   50.770172] RIP: 0010:override_creds+0x19/0x30
...
[   50.770183]  io_worker_handle_work+0x25c/0x430
[   50.770185]  io_wqe_worker+0x2a0/0x350
[   50.770190]  kthread+0x136/0x180
[   50.770194]  ret_from_fork+0x22/0x30
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1791,7 +1791,7 @@ static void io_put_req(struct io_kiocb *req)
 
 static struct io_wq_work *io_steal_work(struct io_kiocb *req)
 {
-	struct io_kiocb *link, *nxt = NULL;
+	struct io_kiocb *nxt = NULL;
 
 	/*
 	 * A ref is owned by io-wq in which context we're. So, if that's the
@@ -1808,10 +1808,15 @@ static struct io_wq_work *io_steal_work(struct io_kiocb *req)
 	if ((nxt->flags & REQ_F_ISREG) && io_op_defs[nxt->opcode].hash_reg_file)
 		io_wq_hash_work(&nxt->work, file_inode(nxt->file));
 
-	link = io_prep_linked_timeout(nxt);
-	if (link)
-		nxt->flags |= REQ_F_QUEUE_TIMEOUT;
-	return &nxt->work;
+	io_req_task_queue(nxt);
+	/*
+	 * If we're going to return actual work, here should be timeout prep:
+	 *
+	 * link = io_prep_linked_timeout(nxt);
+	 * if (link)
+	 *	nxt->flags |= REQ_F_QUEUE_TIMEOUT;
+	 */
+	return NULL;
 }
 
 /*