ipv6: Add disable_ipv6 sysctl to disable IPv6 operaion on specific interface.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 71c7bea..dae980e 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -1025,6 +1025,10 @@
 	autoconfigured addresses.
 	Default: 16
 
+disable_ipv6 - BOOLEAN
+	Disable IPv6 operation.
+	Default: FALSE (enable IPv6 operation)
+
 icmp/*:
 ratelimit - INTEGER
 	Limit the maximal rates for sending ICMPv6 packets.