Dead m_pullup: packetlen=m_len makes check tautologically false — ng7 twin of DF-0555

Summary

ng7 twin of DF-0555. Line :569 packetlen=m->m_len(first mbuf only). Line :576 if(m->m_len<packetlen) always false since same value. m_pullup never invoked. XXX comment :571. Multi-mbuf LMI frames parsed using only first fragment -> valid frames rejected -> false link-down. Fix: packetlen=m_pkthdr.len m_pullup(m,m_pkthdr.len).