io_uring: kiocb_done() should *not* trust ->ki_pos if ->{read,write}_iter() failed

->ki_pos value is unreliable in such cases.  For an obvious example,
consider O_DSYNC write - we feed the data to page cache and start IO,
then we make sure it's completed.  Update of ->ki_pos is dealt with
by the first part; failure in the second ends up with negative value
returned _and_ ->ki_pos left advanced as if sync had been successful.
In the same situation write(2) does not advance the file position
at all.
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

io_uring/rw.c

@@ -339,7 +339,7 @@ static int kiocb_done(struct io_kiocb *req, ssize_t ret,
 	struct io_rw *rw = io_kiocb_to_cmd(req, struct io_rw);
 	unsigned final_ret = io_fixup_rw_res(req, ret);
 
-	if (req->flags & REQ_F_CUR_POS)
+	if (ret >= 0 && req->flags & REQ_F_CUR_POS)
 		req->file->f_pos = rw->kiocb.ki_pos;
 	if (ret >= 0 && (rw->kiocb.ki_complete == io_complete_rw)) {
 		if (!__io_complete_rw_common(req, ret)) {