Fix GRE over IPv6 tunnels with IPFW

Previously, GRE packets in IPv6 tunnels would be dropped by IPFW (unless
net.inet6.ip6.fw.deny_unknown_exthdrs was unset).

PR:		220640
Submitted by:	Kun Xie <kxie@xiplink.com>
MFC after:	1 week

sys/netpfil/ipfw/ip_fw2.c

@@ -92,6 +92,8 @@ __FBSDID("$FreeBSD$");
 #include <netinet6/ip6_var.h>
 #endif
 
+#include <net/if_gre.h> /* for struct grehdr */
+
 #include <netpfil/ipfw/ip_fw_private.h>
 
 #include <machine/in_cksum.h>	/* XXX for in_cksum */
@@ -1162,6 +1164,11 @@ do {								\
 				PULLUP_TO(hlen, ulp, struct pim);
 				break;
 
+			case IPPROTO_GRE:	/* RFC 1701 */
+				/* XXX GRE header check? */
+				PULLUP_TO(hlen, ulp, struct grehdr);
+				break;
+
 			case IPPROTO_CARP:
 				PULLUP_TO(hlen, ulp, struct carp_header);
 				if (((struct carp_header *)ulp)->carp_version !=