netfilter: flowtable: dst_check() from garbage collector path

Move dst_check() to the garbage collector path. Stale routes trigger the
flow entry teardown state which makes affected flows go back to the
classic forwarding path to re-evaluate flow offloading.

IPv6 requires the dst cookie to work, store it in the flow_tuple,
otherwise dst_check() always fails.

Fixes: e5075c0b ("netfilter: flowtable: call dst_check() to fall back to classic forwarding")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>