net/sched: add qstats_cpu_drop_inc() helper

net/sched: add qstats_cpu_drop_inc() helper

1) Using this_cpu_inc() is better than going through this_cpu_ptr():

- Single instruction on x86.
- Store tearing prevention.

2) Change tcf_action_update_stats() to use this_cpu_add().

3) Add WRITE_ONCE() to __qdisc_qstats_drop() and qstats_drop_inc()
   in preparation for lockless "tc qdisc show".

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/0 grow/shrink: 3/17 up/down: 72/-216 (-144)
Function                                     old     new   delta
dualpi2_enqueue_skb                          462     511     +49
tcf_ife_act                                 1061    1077     +16
taprio_enqueue                               613     620      +7
codel_qdisc_enqueue                          149     143      -6
tcf_vlan_act                                 684     676      -8
tcf_skbedit_act                              626     618      -8
tcf_police_act                               725     717      -8
tcf_mpls_act                                1297    1289      -8
tcf_gate_act                                 310     302      -8
tcf_gact_act                                 222     214      -8
tcf_csum_act                                2438    2430      -8
tcf_bpf_act                                  709     701      -8
tcf_action_update_stats                      124     115      -9
pie_qdisc_enqueue                            865     856      -9
pfifo_enqueue                                116     107      -9
choke_enqueue                               2069    2059     -10
plug_enqueue                                 139     128     -11
bfifo_enqueue                                121     110     -11
tcf_nat_act                                 1501    1489     -12
gred_enqueue                                1743    1668     -75
Total: Before=24388609, After=24388465, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20260501135916.2566766-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>