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>