netfilter: SYNPROXY target: restrict to INPUT/FORWARD

Fix a crash in synproxy_send_tcp() when using the SYNPROXY target in the
PREROUTING chain caused by missing routing information.
Reported-by: NNicki P. <xastx@gmx.de>
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/ipt_SYNPROXY.c

@@ -423,6 +423,7 @@ static void synproxy_tg4_destroy(const struct xt_tgdtor_param *par)
 static struct xt_target synproxy_tg4_reg __read_mostly = {
 	.name		= "SYNPROXY",
 	.family		= NFPROTO_IPV4,
+	.hooks		= (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_FORWARD),
 	.target		= synproxy_tg4,
 	.targetsize	= sizeof(struct xt_synproxy_info),
 	.checkentry	= synproxy_tg4_check,

net/ipv6/netfilter/ip6t_SYNPROXY.c

@@ -446,6 +446,7 @@ static void synproxy_tg6_destroy(const struct xt_tgdtor_param *par)
 static struct xt_target synproxy_tg6_reg __read_mostly = {
 	.name		= "SYNPROXY",
 	.family		= NFPROTO_IPV6,
+	.hooks		= (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_FORWARD),
 	.target		= synproxy_tg6,
 	.targetsize	= sizeof(struct xt_synproxy_info),
 	.checkentry	= synproxy_tg6_check,