fs: ReadDirPlus: don't forget overflow entry

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

fs/bridge.go

@@ -858,7 +858,7 @@ func (b *rawBridge) ReadDirPlus(cancel <-chan struct{}, input *fuse.ReadIn, out
 	}
 
 	ctx := &fuse.Context{Caller: input.Caller, Cancel: cancel}
-	for f.dirStream.HasNext() {
+	for f.dirStream.HasNext() || f.hasOverflow {
 		var e fuse.DirEntry
 		var errno syscall.Errno