net/sched: sch_frag: add generic packet fragment support.

Currently kernel tc subsystem can do conntrack in cat_ct. But when several
fragment packets go through the act_ct, function tcf_ct_handle_fragments
will defrag the packets to a big one. But the last action will redirect
mirred to a device which maybe lead the reassembly big packet over the mtu
of target device.

This patch add support for a xmit hook to mirred, that gets executed before
xmiting the packet. Then, when act_ct gets loaded, it configs that hook.
The frag xmit hook maybe reused by other modules.

Signed-off-by: wenxu <wenxu@ucloud.cn>
Acked-by: Cong Wang <cong.wang@bytedance.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
diff --git a/net/sched/act_ct.c b/net/sched/act_ct.c
index aba3cd8..61092c5 100644
--- a/net/sched/act_ct.c
+++ b/net/sched/act_ct.c
@@ -1541,6 +1541,8 @@ static int __init ct_init_module(void)
 	if (err)
 		goto err_register;
 
+	static_branch_inc(&tcf_frag_xmit_count);
+
 	return 0;
 
 err_register:
@@ -1552,6 +1554,7 @@ static int __init ct_init_module(void)
 
 static void __exit ct_cleanup_module(void)
 {
+	static_branch_dec(&tcf_frag_xmit_count);
 	tcf_unregister_action(&act_ct_ops, &ct_net_ops);
 	tcf_ct_flow_tables_uninit();
 	destroy_workqueue(act_ct_wq);