tmpfiles: use fd_get_path() less excessively

tmpfiles: use fd_get_path() less excessively

fd_get_path() is an ugly API, as it creates ambiguities related to the
" (deleted)" suffix /proc/$PID/fd/$FD shows. Let's use it a bit less
excessively, and whenever we have a good valid path already, let's
simply pass that along, instead of forgetting it in one stackframe and
reacquiring it in the next.