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: Bijan Mottahedeh <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

fs/io_uring.c

···
 	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;
···
 	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;