Add a new macro M_ASSERTVALID which ensures that the mbuf in question

is non-free.  (More checks can/should be added in the future.)

Use M_ASSERTVALID in BPF_MTAP so that we catch when freed mbufs are
passed in, even if no bpf listeners are active.

Inspired by a bug in if_dc caught by Kenjiro Cho.

sys/net/bpf.h

@@ -366,6 +366,7 @@ u_int	 bpf_filter(const struct bpf_insn *, u_char *, u_int, u_int);
 		bpf_tap((_ifp)->if_bpf, (_pkt), (_pktlen));	\
 } while (0)
 #define	BPF_MTAP(_ifp,_m) do {					\
+	M_ASSERTVALID(_m);					\
 	if ((_ifp)->if_bpf)					\
 		bpf_mtap((_ifp)->if_bpf, (_m));			\
 } while (0)

sys/sys/mbuf.h

@@ -339,6 +339,13 @@ struct mbstat {
 	KASSERT(m != NULL && m->m_flags & M_PKTHDR,	\
 		("%s: no mbuf packet header!", __func__))
 
+/*
+ * Ensure that the supplied mbuf is a valid, non-free mbuf.
+ */
+#define M_ASSERTVALID(m)					\
+	KASSERT((((struct mbuf *)m)->m_flags & M_FREELIST) == 0,			\
+		("%s: attempted use of a free mbuf!", __func__))
+
 /*
  * Set the m_data pointer of a newly-allocated mbuf (m_get/MGET) to place
  * an object of the specified size at the end of the mbuf, longword aligned.