* do not c/r mc_forwarding option it depends on multicast
managment socket existence and is readonly.

In addrconf_sysctl_disable->addrconf_disable_ipv6->dev_disable_change:
On addrconf_notify + NETDEV_UP if idev->cnf.mtu6 differs from dev->mtu,
sysctl mtu6 is overwritten.

So changing disable_ipv6 sysctl for some device may change mtu sysctl
for it and we need to restore disable_ipv6 first and only then mtu.
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Reviewed-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>