[PATCH v2] tcp: update outdated comment for removed _decode_session6()

[Kexin Sun]

The comment in tcp_v6_fill_cb() states that the memmove of
IP6CB into TCP_SKB_CB must be done after xfrm6_policy_check()
because _decode_session6() uses IP6CB().

This ordering constraint was introduced by commit 2dc49d1680b5
("tcp6: don't move IP6CB before xfrm6_policy_check()") and was
valid at the time.  However, commit 7a0207094f1b ("xfrm: policy:
replace session decode with flow dissector") replaced the
per-family decode_session callbacks with the flow dissector,
which parses packet headers directly without accessing IP6CB().

Since xfrm6_policy_check() no longer reads IP6CB() anywhere in
its call chain, the ordering constraint no longer applies.
Simplify the comment to describe only what the memmove and
barrier() actually do.

Assisted-by: unnamed:deepseek-v3.2 coccinelle
Suggested-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
Signed-off-by: Kexin Sun <kexinsun@xxxxxxxxxxxxxxxx>
---
 net/ipv6/tcp_ipv6.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index d10487b4e5bf..619b35669361 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1688,10 +1688,9 @@ int tcp_v6_do_rcv(struct sock *sk, struct sk_buff *skb)
 static void tcp_v6_fill_cb(struct sk_buff *skb, const struct ipv6hdr *hdr,
 			   const struct tcphdr *th)
 {
-	/* This is tricky: we move IP6CB at its correct location into
-	 * TCP_SKB_CB(). It must be done after xfrm6_policy_check(), because
-	 * _decode_session6() uses IP6CB().
-	 * barrier() makes sure compiler won't play aliasing games.
+	/* Move IP6CB to its saved location in TCP_SKB_CB().
+	 * barrier() makes sure compiler won't play aliasing
+	 * games.
 	 */
 	memmove(&TCP_SKB_CB(skb)->header.h6, IP6CB(skb),
 		sizeof(struct inet6_skb_parm));
-- 
2.25.1