udp: do not use skb_release_head_state() before skb_attempt_defer_free()

Michal reported and bisected an issue after recent adoption
of skb_attempt_defer_free() in UDP.

The issue here is that skb_release_head_state() is called twice per skb,
one time from skb_consume_udp(), then a second time from skb_defer_free_flush()
and napi_consume_skb().

As Sabrina suggested, remove skb_release_head_state() call from
skb_consume_udp().

Add a DEBUG_NET_WARN_ON_ONCE(skb_nfct(skb)) in skb_attempt_defer_free()

Many thanks to Michal, Sabrina, Paolo and Florian for their help.

Fixes: 6471658 ("udp: use skb_attempt_defer_free()")
Reported-and-bisected-by: Michal Kubecek <mkubecek@suse.cz>
Closes: https://lore.kernel.org/netdev/gpjh4lrotyephiqpuldtxxizrsg6job7cvhiqrw72saz2ubs3h@g6fgbvexgl3r/
Signed-off-by: Eric Dumazet <edumazet@google.com>
Tested-by: Michal Kubecek <mkubecek@suse.cz>
Cc: Sabrina Dubroca <sd@queasysnail.net>
Cc: Florian Westphal <fw@strlen.de>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20251015052715.4140493-1-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: edumazet

net/core/skbuff.c

@@ -7200,6 +7200,7 @@ nodefer:	kfree_skb_napi_cache(skb);
 
 	DEBUG_NET_WARN_ON_ONCE(skb_dst(skb));
 	DEBUG_NET_WARN_ON_ONCE(skb->destructor);
+	DEBUG_NET_WARN_ON_ONCE(skb_nfct(skb));
 
 	sdn = per_cpu_ptr(net_hotdata.skb_defer_nodes, cpu) + numa_node_id();
 

net/ipv4/udp.c

@@ -1851,8 +1851,6 @@ void skb_consume_udp(struct sock *sk, struct sk_buff *skb, int len)
 		sk_peek_offset_bwd(sk, len);
 
 	if (!skb_shared(skb)) {
-		if (unlikely(udp_skb_has_head_state(skb)))
-			skb_release_head_state(skb);
 		skb_attempt_defer_free(skb);
 		return;
 	}