fdio: Delete .tmp file on failure

We noticed the temp files being left over in ostree when (mistakenly)
trying to replace the contents of a subpath that wasn't a directory.

In the future we should look at the systemd code using `O_TMPFILE`
here.

glnx-fdio.c

@@ -746,6 +746,8 @@ glnx_file_replace_contents_with_perms_at (int                   dfd,
 
   ret = TRUE;
  out:
+  if (!ret)
+    (void) unlink (tmppath);
   return ret;
 }