ipv6: Initial skb->dev and skb->protocol in ip6_output

Move the initialization of skb->dev and skb->protocol from
ip6_finish_output2 to ip6_output. This can make the skb->dev and
skb->protocol information avalaible to the CGROUP eBPF filter.

Signed-off-by: Chenbo Feng <fengc@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>