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: 6471658dc66c ("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>

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