gve: Clear napi->skb before dev_kfree_skb_any()

In the Linux kernel, the following vulnerability has been resolved: gve: Clear napi->skb before dev_kfree_skb_any() gve_rx_free_skb incorrectly leaves napi->skb referencing an skb after it is freed with dev_kfree_skb_any(). This can result in a subsequent call to napi_get_frags returning a dangling pointer. Fix this by clearing napi->skb before the skb is freed.