Remove the redundant skb->dev initialization in ip6_fragment

After moves the skb->dev and skb->protocol initialization into
ip6_output, setting the skb->dev inside ip6_fragment is unnecessary.

Fixes: 97a7a37a("ipv6: Initial skb->dev and skb->protocol in ip6_output")
Signed-off-by: Chenbo Feng <fengc@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>