CVE-2024-40937 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.