Even when the directory stream does not have additional
entries, we may still have an entry in the overflow slot.

Don't forget about it. Test now passes:

  go-fuse/fs$ go test
  PASS
  ok  	github.com/hanwen/go-fuse/v2/fs	0.582s

Change-Id: I98b39d4e7df235ed57ae4bf2702532f3b17d1bd0