netfilter: flowtable: allow unidirectional rules

[ Upstream commit 8f84780b ]

Modify flow table offload to support unidirectional connections by
extending enum nf_flow_flags with new "NF_FLOW_HW_BIDIRECTIONAL" flag. Only
offload reply direction when the flag is set. This infrastructure change is
necessary to support offloading UDP NEW connections in original direction
in following patches in series.

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 125f1c7f ("net/sched: act_ct: Take per-cb reference to tcf_ct_flow_table")
Signed-off-by: Sasha Levin <sashal@kernel.org>