ipv6: Disable flowlabel state ranges by default

Per RFC6437 stateful flow labels (e.g. labels set by flow label manager)
cannot "disturb" nodes taking part in stateless flow labels. While the
ranges only reduce the flow label entropy by one bit, it is conceivable
that this might bias the algorithm on some routers causing a load
imbalance. For best results on the Internet we really need the full
20 bits.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>