ipv6: Implement different admin modes for automatic flow labels

Change the meaning of net.ipv6.auto_flowlabels to provide a mode for
automatic flow labels generation. There are four modes:

0: flow labels are disabled
1: flow labels are enabled, sockets can opt-out
2: flow labels are allowed, sockets can opt-in
3: flow labels are enabled and enforced, no opt-out for sockets

np->autoflowlabel is initialized according to the sysctl value.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index 54e694c..b0ab420 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -1095,7 +1095,7 @@
 	skb_reset_network_header(skb);
 	ipv6h = ipv6_hdr(skb);
 	ip6_flow_hdr(ipv6h, INET_ECN_encapsulate(0, dsfield),
-		     ip6_make_flowlabel(net, skb, fl6->flowlabel, false, fl6));
+		     ip6_make_flowlabel(net, skb, fl6->flowlabel, true, fl6));
 	ipv6h->hop_limit = t->parms.hop_limit;
 	ipv6h->nexthdr = proto;
 	ipv6h->saddr = fl6->saddr;