[PATCH] tcp: update outdated comment for removed _decode_session6()

[Kexin Sun]

The function _decode_session6() was moved to
net/xfrm/xfrm_policy.c and renamed to decode_session6() by commit
c53ac41e3720 ("xfrm: remove decode_session indirection from
afinfo_policy"), and was later refactored to use flow dissector
instead of accessing IP6CB() directly by commit 7a0207094f1b
("xfrm: policy: replace session decode with flow dissector").

Replace the stale function reference with a general description of
the ordering constraint.

Assisted-by: unnamed:deepseek-v3.2 coccinelle
Signed-off-by: Kexin Sun <kexinsun@xxxxxxxxxxxxxxxx>
---
 net/ipv6/tcp_ipv6.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index d10487b4e5bf..84d95e76085a 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1690,7 +1690,7 @@ static void tcp_v6_fill_cb(struct sk_buff *skb, const struct ipv6hdr *hdr,
 {
 	/* 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().
+	 * the flow decoding during XFRM policy check uses IP6CB().
 	 * barrier() makes sure compiler won't play aliasing games.
 	 */
 	memmove(&TCP_SKB_CB(skb)->header.h6, IP6CB(skb),
-- 
2.25.1