netfilter: flowtable: fetch stats only if flow is still alive

Do not fetch statistics if flow has expired since it might not in
hardware anymore. After this update, remove the FLOW_OFFLOAD_HW_DYING
check from nf_flow_offload_stats() since this flag is never set on.

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