netfilter: flowtable_offload: fix using __this_cpu_add in preemptible

flow_offload_queue_work() can be called in workqueue without
bh disabled, like the call trace showed in my act_ct testing,
calling NF_FLOW_TABLE_STAT_INC() there would cause a call
trace:

  BUG: using __this_cpu_add() in preemptible [00000000] code: kworker/u4:0/138560
  caller is flow_offload_queue_work+0xec/0x1b0 [nf_flow_table]
  Workqueue: act_ct_workqueue tcf_ct_flow_table_cleanup_work [act_ct]
  Call Trace:
   <TASK>
   dump_stack_lvl+0x33/0x46
   check_preemption_disabled+0xc3/0xf0
   flow_offload_queue_work+0xec/0x1b0 [nf_flow_table]
   nf_flow_table_iterate+0x138/0x170 [nf_flow_table]
   nf_flow_table_free+0x140/0x1a0 [nf_flow_table]
   tcf_ct_flow_table_cleanup_work+0x2f/0x2b0 [act_ct]
   process_one_work+0x6a3/0x1030
   worker_thread+0x8a/0xdf0

This patch fixes it by using NF_FLOW_TABLE_STAT_INC_ATOMIC()
instead in flow_offload_queue_work().

Note that for FLOW_CLS_REPLACE branch in flow_offload_queue_work(),
it may not be called in preemptible path, but it's good to use
NF_FLOW_TABLE_STAT_INC_ATOMIC() for all cases in
flow_offload_queue_work().

Fixes: b0381776 ("netfilter: nf_flow_table: count pending offload workqueue tasks")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_flow_table_offload.c

@@ -997,13 +997,13 @@ static void flow_offload_queue_work(struct flow_offload_work *offload)
 	struct net *net = read_pnet(&offload->flowtable->net);
 
 	if (offload->cmd == FLOW_CLS_REPLACE) {
-		NF_FLOW_TABLE_STAT_INC(net, count_wq_add);
+		NF_FLOW_TABLE_STAT_INC_ATOMIC(net, count_wq_add);
 		queue_work(nf_flow_offload_add_wq, &offload->work);
 	} else if (offload->cmd == FLOW_CLS_DESTROY) {
-		NF_FLOW_TABLE_STAT_INC(net, count_wq_del);
+		NF_FLOW_TABLE_STAT_INC_ATOMIC(net, count_wq_del);
 		queue_work(nf_flow_offload_del_wq, &offload->work);
 	} else {
-		NF_FLOW_TABLE_STAT_INC(net, count_wq_stats);
+		NF_FLOW_TABLE_STAT_INC_ATOMIC(net, count_wq_stats);
 		queue_work(nf_flow_offload_stats_wq, &offload->work);
 	}
 }