ipv6: add flowlabel_consistency sysctl

With the introduction of IPV6_FL_F_REFLECT, there is no guarantee of
flow label unicity. This patch introduces a new sysctl to protect the old
behaviour, enable by default.

Changelog of V3:
 * rename ip6_flowlabel_consistency to flowlabel_consistency
 * use net_info_ratelimited()
 * checkpatch cleanups

Signed-off-by: Florent Fourcot <florent.fourcot@enst-bretagne.fr>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
index c921d5d..d935889 100644
--- a/net/ipv6/af_inet6.c
+++ b/net/ipv6/af_inet6.c
@@ -775,6 +775,7 @@
 
 	net->ipv6.sysctl.bindv6only = 0;
 	net->ipv6.sysctl.icmpv6_time = 1*HZ;
+	net->ipv6.sysctl.flowlabel_consistency = 1;
 	atomic_set(&net->ipv6.rt_genid, 0);
 
 	err = ipv6_init_mibs(net);