kernel: veth: ensure skb entering GRO are not cloned.

In the Linux kernel, the following vulnerability has been resolved: veth: ensure skb entering GRO are not cloned. After commit d3256efd8e8b "veth: allow enabling NAPI even without XDP", if GRO is enabled on a veth device and TSO is disabled on the peer device, TCP skbs will go through the NAPI...