Defer the fragmentation hash secret initialization for IPv6 like the
previous patch did for IPv4.

Because the netfilter logic reuses the hash secret we have to split it
first. Thus introduce a new nf_hash_frag function which takes care to
seed the hash secret.

Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>