fuse_ops: fix a few small bugs
fuse_ops: fix a few small bugs

* Only canonical folder strings should get a listed top-node.
* Avoid segfault when stat'ing/listing root dir by just pretending it
  has DirID == 0

keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client