ipv6: enable net.ipv6.route.max_size sysctl in network namespace

We want to increase route cache size in network namespace
created with user namespace. Currently ipv6 route settings
are disabled for non-initial network namespaces.
We can allow this sysctl and it will be safe since
commit <6126891c> because route cache account to kmem,
that is why users from user namespace can not DOS system.

Signed-off-by: Alexander Kuznetsov <wwfq@yandex-team.ru>
Acked-by: Dmitry Yakunin <zeil@yandex-team.ru>
Acked-by: Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>