This driver attempts to cache mbufs, and re-use them as headers instead

of using the MGETHDR macro all the time.  When an mbuf is reused as a
header, initialize csum_flags to zero as well, so the delayed_checksum
call woks properly.

Debbugging work done by: jmas

sys/dev/vx/if_vx.c

@@ -822,6 +822,7 @@ vxget(sc, totlen)
         sc->next_mb = (sc->next_mb + 1) % MAX_MBS;
         m->m_data = m->m_pktdat;
         m->m_flags = M_PKTHDR;
+        m->m_pkthdr.csum_flags = 0;
     }
     m->m_pkthdr.rcvif = ifp;
     m->m_pkthdr.len = totlen;