[IPV6]: Disallow RH0 by default.

A security issue is emerging.  Disallow Routing Header Type 0 by default
as we have been doing for IPv4.
Note: We allow RH2 by default because it is harmless.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -851,6 +851,15 @@ accept_redirects - BOOLEAN
 	Functional default: enabled if local forwarding is disabled.
 			    disabled if local forwarding is enabled.
 
+accept_source_route - INTEGER
+	Accept source routing (routing extension header).
+
+	> 0: Accept routing header.
+	= 0: Accept only routing header type 2.
+	< 0: Do not accept routing header.
+
+	Default: 0
+
 autoconf - BOOLEAN
 	Autoconfigure addresses using Prefix Information in Router 
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