netfilter: nf_tables: fix bidirectional offload regression

Commit 8f84780b ("netfilter: flowtable: allow unidirectional rules")
made unidirectional flow offload possible, while completely ignoring (and
breaking) bidirectional flow offload for nftables.
Add the missing flag that was left out as an exercise for the reader :)

Cc: Vlad Buslov <vladbu@nvidia.com>
Fixes: 8f84780b ("netfilter: flowtable: allow unidirectional rules")
Reported-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>