Assert that the mbuf is not shared to ensure problems like the last commit are

not reintroduced.

sys/net/if_bridge.c

@@ -2589,6 +2589,9 @@ bridge_pfil(struct mbuf **mp, struct ifnet *bifp, struct ifnet *ifp, int dir)
 	snap = 0;
 	error = -1;	/* Default error if not error == 0 */
 
+	/* we may return with the IP fields swapped, ensure its not shared */
+	KASSERT(M_WRITABLE(*mp), ("%s: modifying a shared mbuf", __func__));
+
 	if (pfil_bridge == 0 && pfil_member == 0 && pfil_ipfw == 0)
 		return 0; /* filtering is disabled */