fou: remove warn in gue_gro_receive on unsupported protocol

Drop the WARN_ON_ONCE inn gue_gro_receive if the encapsulated type is
not known or does not have a GRO handler.

Such a packet is easily constructed. Syzbot generates them and sets
off this warning.

Remove the warning as it is expected and not actionable.

The warning was previously reduced from WARN_ON to WARN_ON_ONCE in
commit 27013661 ("fou: Do WARN_ON_ONCE in gue_gro_receive for bad
proto callbacks").
Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240614122552.1649044-1-willemdebruijn.kernel@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/fou_core.c

@@ -433,7 +433,7 @@ static struct sk_buff *gue_gro_receive(struct sock *sk,
 
 	offloads = NAPI_GRO_CB(skb)->is_ipv6 ? inet6_offloads : inet_offloads;
 	ops = rcu_dereference(offloads[proto]);
-	if (WARN_ON_ONCE(!ops || !ops->callbacks.gro_receive))
+	if (!ops || !ops->callbacks.gro_receive)
 		goto out;
 
 	pp = call_gro_receive(ops->callbacks.gro_receive, head, skb);