net/sched: sch_cake: drop WARN_ON(1) for malformed packets in ACK filter

The sch_cake ACK filter parses packets to find the TCP header and filter
duplicated ACKs if the flow is backlogged. The parsing code contains a
WARN_ON(1) which can be triggered by a malformed IP header in certain
cases. Depending on the system configuration, this leads either to
either spamming dmesg with warnings, or a panic if panic_on_warn is set.

The code already correctly skips the offending packet in the branch that
triggers the warning, so the WARN_ON itself doesn't really serve any
purpose. So just drop it altogether to avoid the inconvenient side
effects.

Fixes: 8b7138814f ("sch_cake: Add optional ACK filter")
Reported-by: Zhiling Zou <zhilinz@nebusec.ai>
Reported-by: Ren Wei <enjou1224z@gmail.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://patch.msgid.link/20260729191417.45665-1-toke@toke.dk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/sched/sch_cake.c

@@ -1287,7 +1287,6 @@ static struct sk_buff *cake_ack_filter(struct cake_sched_data *q,
 
 			seglen = ipv6_payload_len(skb, ipv6h_check);
 		} else {
-			WARN_ON(1);  /* shouldn't happen */
 			continue;
 		}