netfilter: ipv6: Fix undefined symbol nf_ct_frag6_gather

CONFIG_NETFILTER=m and CONFIG_NF_DEFRAG_IPV6 is not set

ERROR: "nf_ct_frag6_gather" [net/ipv6/ipv6.ko] undefined!

Fixes: c9bb6165 ("netfilter: nf_conntrack_bridge: fix CONFIG_IPV6=y")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv6/netfilter.c

@@ -238,8 +238,10 @@ static const struct nf_ipv6_ops ipv6ops = {
 	.route_input		= ip6_route_input,
 	.fragment		= ip6_fragment,
 	.reroute		= nf_ip6_reroute,
-#if IS_MODULE(CONFIG_IPV6)
+#if IS_MODULE(CONFIG_IPV6) && IS_ENABLED(CONFIG_NF_DEFRAG_IPV6)
 	.br_defrag		= nf_ct_frag6_gather,
+#endif
+#if IS_MODULE(CONFIG_IPV6)
 	.br_fragment		= br_ip6_fragment,
 #endif
 };