netns: igmp: allow IPPROTO_IGMP sockets in netns

Looks like everything is already ready.

Required for ebtables(8) for one thing.

Also, required for ipmr per-netns (coming soon). (Benjamin)
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/af_inet.c

@@ -1402,6 +1402,7 @@ EXPORT_SYMBOL_GPL(snmp_mib_free);
 #ifdef CONFIG_IP_MULTICAST
 static struct net_protocol igmp_protocol = {
 	.handler =	igmp_rcv,
+	.netns_ok =	1,
 };
 #endif