netfilter: bridge: Drops IPv6 packets if IPv6 module is not loaded

A kernel panic can happen if a host has disabled IPv6 on boot and have to
process guest packets (coming from a bridge) using it's ip6tables.

IPv6 packets need to be dropped if the IPv6 module is not loaded, and the
host ip6tables will be used.
Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/bridge/br_netfilter_hooks.c

@@ -496,6 +496,10 @@ static unsigned int br_nf_pre_routing(void *priv,
 		if (!brnet->call_ip6tables &&
 		    !br_opt_get(br, BROPT_NF_CALL_IP6TABLES))
 			return NF_ACCEPT;
+		if (!ipv6_mod_enabled()) {
+			pr_warn_once("Module ipv6 is disabled, so call_ip6tables is not supported.");
+			return NF_DROP;
+		}
 
 		nf_bridge_pull_encap_header_rcsum(skb);
 		return br_nf_pre_routing_ipv6(priv, skb, state);