rgw_file: fix invalidation of top-level directories
Originally, root_fh was reachable from RGWLibFS::fh_cache, but
this was problematic as it has infinite lifetime and so is not
present in RGWLibFS::fs_lru.
To fix the above, RGWLibFS::root_fh came to live unhooked from the
handle caches which works because nfs-ganesha/the application
always has an instance handle. However, this silently breaks
READDIR event invalidation.