net: use kfree_skb_list() from ip_do_fragment()

Just like 46cfd725 ("net: use kfree_skb_list() helper in more places").

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>