netfilter: nf_conntrack_ipv6: Plug sk_buff leak in fragment handling

In commit 4cdd3408 ("netfilter: nf_conntrack_ipv6: improve fragmentation
handling"), an sk_buff leak was introduced when dealing with reassembled
packets by grabbing a reference to the original skb instead of the
reassembled skb.  At this point, the leak only impacted conntracks with an
associated helper.

In commit 58a317f1 ("netfilter: ipv6: add IPv6 NAT support"), the bug was
expanded to include all reassembled packets with unconfirmed conntracks.

Fix this by grabbing a reference to the proper reassembled skb.  This
closes netfilter bugzilla #823.

Signed-off-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>