netfilter: nf_dup_ipv6: set again FLOWI_FLAG_KNOWN_NH at flowi6_flags

netfilter: nf_dup_ipv6: set again FLOWI_FLAG_KNOWN_NH at flowi6_flags

With the commit e29fcf1c5e3a ("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 b9d5880b36ae ("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: b9d5880b36ae ("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>