netfilter: nf_ct_reasm: fix conntrack reassembly expire code

Commit b836c99fd6c9 (ipv6: unify conntrack reassembly expire
code with standard one) use the standard IPv6 reassembly
code(ip6_expire_frag_queue) to handle conntrack reassembly expire.

In ip6_expire_frag_queue, it invoke dev_get_by_index_rcu to get
which device received this expired packet.so we must save ifindex
when NF_conntrack get this packet.

With this patch applied, I can see ICMP Time Exceeded sent
from the receiver when the sender sent out 1/2 fragmented
IPv6 packet.

Signed-off-by: Haibo Xi <haibbo@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>