When we are about to send down to the driver layer

we need to make sure that the m_nextpkt field is NULL
else the lower layers may do unwanted things.

Approved by:	re(gjb)
Reviewed By:  gallatin, melifaro
Differential Revision: https://reviews.freebsd.org/D28377

(cherry picked from commit 538a815)

sys/netinet6/nd6.c

@@ -2460,6 +2460,7 @@ nd6_flush_holdchain(struct ifnet *ifp, struct mbuf *chain,
 	while (m_head) {
 		m = m_head;
 		m_head = m_head->m_nextpkt;
+		m->m_nextpkt = NULL;
 		error = nd6_output_ifp(ifp, ifp, m, dst, NULL);
 	}