netfilter: arp_tables: register table in initns

arptables is broken since we didn't register the table anymore --
even 'arptables -L' fails.

Fixes: b9e69e127397187b ("netfilter: xtables: don't hook tables by default")
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/ipv4/netfilter/arptable_filter.c

···
 		return ret;
 	}
 
+	ret = arptable_filter_table_init(&init_net);
+	if (ret) {
+		unregister_pernet_subsys(&arptable_filter_net_ops);
+		kfree(arpfilter_ops);
+	}
+
 	return ret;
 }