netfilter: nf_dup_ipv6: set again FLOWI_FLAG_KNOWN_NH at flowi6_flags

commit 83170f3b upstream.

With the commit 48e8aa6e ("ipv6: Set FLOWI_FLAG_KNOWN_NH at
flowi6_flags") ip6_pol_route() callers were asked to to set the
FLOWI_FLAG_KNOWN_NH properly and xt_TEE was updated accordingly,
but with the later refactor in commit bbde9fc1 ("netfilter:
factor out packet duplication for IPv4/IPv6") the flowi6_flags
update was lost.
This commit re-add it just before the routing decision.

Fixes: bbde9fc1 ("netfilter: factor out packet duplication for IPv4/IPv6")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>