ipv6: Register some net/ipv6/ core sysctls at read-only root.

There are some sysctls left to be switched to read-only,
but they are all in ipv6, so complete with them.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>