vhost: fix log ctx signalling

The log eventfd signalling got put in dead code.
We didn't notice because qemu currently does polling
instead of eventfd select.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/vhost.c

@@ -858,11 +858,12 @@ int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
 		if (r < 0)
 			return r;
 		len -= l;
-		if (!len)
-			return 0;
-	}
+		if (!len) {
 			if (vq->log_ctx)
 				eventfd_signal(vq->log_ctx, 1);
+			return 0;
+		}
+	}
 	/* Length written exceeds what we have stored. This is a bug. */
 	BUG();
 	return 0;