[bug report] io_uring: add support for sqe links

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Jens Axboe,

The patch f3fafe4103bd: "io_uring: add support for sqe links" from
May 10, 2019, leads to the following static checker warning:

	fs/io_uring.c:623 io_req_link_next()
	error: potential NULL dereference 'nxt'.

fs/io_uring.c
   614  static void io_req_link_next(struct io_kiocb *req)
   615  {
   616          struct io_kiocb *nxt;
   617  
   618          nxt = list_first_entry_or_null(&req->link_list, struct io_kiocb, list);
   619          list_del(&nxt->list);
                          ^^^^^^^^^
The warning is a false positive but this is a NULL dereference.

   620          if (!list_empty(&req->link_list)) {
   621                  INIT_LIST_HEAD(&nxt->link_list);
                                        ^^^^^
False positive.

   622                  list_splice(&req->link_list, &nxt->link_list);
   623                  nxt->flags |= REQ_F_LINK;
   624          }
   625  
   626          INIT_WORK(&nxt->work, io_sq_wq_submit_work);
                          ^^^^^^^^^^
   627          queue_work(req->ctx->sqo_wq, &nxt->work);
                                             ^^^^^^^^^^
Other bugs.

   628  }

regards,
dan carpenter