ipv6: Call skb_get_hash_flowi6 to get skb->hash in ip6_make_flowlabel

We can't call skb_get_hash here since the packet is not complete to do
flow_dissector. Create hash based on flowi6 instead.

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