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 <[e-mail]>

drivers/vhost/vhost.c

···
 		if (r < 0)
 			return r;
 		len -= l;
-		if (!len)
+		if (!len) {
+			if (vq->log_ctx)
+				eventfd_signal(vq->log_ctx, 1);
 			return 0;
+		}
 	}
-	if (vq->log_ctx)
-		eventfd_signal(vq->log_ctx, 1);
 	/* Length written exceeds what we have stored. This is a bug. */
 	BUG();
 	return 0;