netfilter: implement RFC3168 5.3 (ecn protection) for ipv6 fragmentation handling

This change brings netfilter reassembly logic on par with
reassembly.c. The corresponding change in net-next is
(eec2e61 ipv6: implement RFC3168 5.3 (ecn protection) for
ipv6 fragmentation handling)

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Jesper Dangaard Brouer <jbrouer@redhat.com>
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>