netfilter: Use flowlabel flow key when re-routing mangled packets

'ip6 dscp set $v' in an nftables outpute route chain has no effect.
While nftables does detect the dscp change and calls the reroute hook.
But ip6_route_me_harder never sets the dscp/flowlabel:
flowlabel/dsfield routing rules are ignored and no reroute takes place.

Thanks to Yi Chen for an excellent reproducer script that I used
to validate this change.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Reported-by: Yi Chen <yiche@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>