netfilter: nft_{fwd,dup}_netdev: add offload support

This patch adds support for packet mirroring and redirection. The
nft_fwd_dup_netdev_offload() function configures the flow_action object
for the fwd and the dup actions.

Extend nft_flow_rule_destroy() to release the net_device object when the
flow_rule object is released, since nft_fwd_dup_netdev_offload() bumps
the net_device reference counter.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: wenxu <wenxu@ucloud.cn>