netfilter: nf_flow_table_offload: fix incorrect ethernet dst address

Ethernet destination for original traffic takes the source ethernet address
in the reply direction. For reply traffic, this takes the source
ethernet address of the original direction.

Fixes: c29f74e0 ("netfilter: nf_flow_table: hardware offload support")
Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>