netfilter: nf_flow_table_offload: fix the nat port mangle.

Shift on 32-bit word to define the port number depends on the flow
direction.

Fixes: c29f74e0 ("netfilter: nf_flow_table: hardware offload support")
Fixes: 7acd9378

 ("netfilter: nf_flow_table_offload: Correct memcpy size for flow_overload_mangle()")
Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>