io_uring: clear req->result always before issuing a read/write request

req->result is cleared when io_issue_sqe() calls io_read/write_pre()
routines.  Those routines however are not called when the sqe
argument is NULL, which is the case when io_issue_sqe() is called from
io_wq_submit_work().  io_issue_sqe() may then examine a stale result if
a polled request had previously failed with -EAGAIN:

        if (ctx->flags & IORING_SETUP_IOPOLL) {
                if (req->result == -EAGAIN)
                        return -EAGAIN;

                io_iopoll_req_issued(req);
        }

and in turn cause a subsequently completed request to be re-issued in
io_wq_submit_work().
Signed-off-by: NBijan Mottahedeh <bijan.mottahedeh@oracle.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1843,6 +1843,7 @@ static int io_read(struct io_kiocb *req, struct io_kiocb **nxt,
 	if (!force_nonblock)
 		req->rw.kiocb.ki_flags &= ~IOCB_NOWAIT;
 
+	req->result = 0;
 	io_size = ret;
 	if (req->flags & REQ_F_LINK)
 		req->result = io_size;
@@ -1930,6 +1931,7 @@ static int io_write(struct io_kiocb *req, struct io_kiocb **nxt,
 	if (!force_nonblock)
 		req->rw.kiocb.ki_flags &= ~IOCB_NOWAIT;
 
+	req->result = 0;
 	io_size = ret;
 	if (req->flags & REQ_F_LINK)
 		req->result = io_size;