M_COPYFLAGS
to->m_flags |= from->m_flags & M_COPYFLAGS;
to->m_flags = (from->m_flags & M_COPYFLAGS) |
(to->m_flags & ~M_COPYFLAGS);
#define MBUF_CLEARFLAGS (M_COPYFLAGS & ~(M_PKTHDR | M_EOR | M_PRIO))
m->m_flags = m0->m_flags & M_COPYFLAGS;