fixed a bug that MLD-compat-mode never switches back to MLDv2 from MLDv1-compatible mode
fixed a bug that MLD-compat-mode never switches back to MLDv2
from MLDv1-compatible mode

MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame