Fix locking with filter. This changes the way objects are looked up in

Fix locking with filter.  This changes the way objects are looked up in
the filter to always use lookup_one_len() from the parent (which is gotten
at filter_setup time) instead of doing a file open each time.

There appears to be some sort of leak which prevents the loop device from
being cleaned up, but e.g. bonnie runs just fine and all of the modules
can be unloaded.  Will look at it again later once I've gotten some more
urgent things out of the way.