Remove NetBSD'isms (add FreeBSD'isms?), which makes gre(4) working again.

sys/net/if_gre.c

@@ -351,8 +351,7 @@ gre_output(struct ifnet *ifp, struct mbuf *m, struct sockaddr *dst,
 	gh = mtod(m, struct greip *);
 	if (sc->g_proto == IPPROTO_GRE) {
 		/* we don't have any GRE flags for now */
-
-		memset((void *)&gh->gi_g, 0, sizeof(struct gre_h));
+		memset((void *)gh, 0, sizeof(struct greip));
 		gh->gi_ptype = htons(etype);
 	}
 
@@ -364,7 +363,7 @@ gre_output(struct ifnet *ifp, struct mbuf *m, struct sockaddr *dst,
 		((struct ip*)gh)->ip_ttl = GRE_TTL;
 		((struct ip*)gh)->ip_tos = ip->ip_tos;
 		((struct ip*)gh)->ip_id = ip->ip_id;
-		gh->gi_len = htons(m->m_pkthdr.len);
+		gh->gi_len = m->m_pkthdr.len;
 	}
 
 	ifp->if_opackets++;

sys/netinet/ip_gre.c

@@ -204,8 +204,8 @@ gre_input2(struct mbuf *m ,int hlen, u_char proto)
 		m_freem(m);
 		return (EINVAL);
 	}
+	/* Unlike NetBSD, in FreeBSD m_adj() adjusts m->m_pkthdr.len as well */
 	m_adj(m, hlen);
-	m->m_pkthdr.len -= hlen;
 
 	if (sc->sc_if.if_bpf) {
 		u_int32_t af = AF_INET;