pipe: fix failure to return error code on ->confirm()

The arguments were transposed, we want to assign the error code to
'ret', which is being returned.
Signed-off-by: Nicolas Kaiser <nikai@nikai.net>
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

fs/pipe.c

@@ -382,7 +382,7 @@ pipe_read(struct kiocb *iocb, const struct iovec *_iov,
 			error = ops->confirm(pipe, buf);
 			if (error) {
 				if (!ret)
-					error = ret;
+					ret = error;
 				break;
 			}