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.

Fixes: 9b8dd5e5

 ("gve: DQO: Add RX path")
Cc: stable@vger.kernel.org
Reported-by: Shailend Chand <shailend@google.com>
Signed-off-by: Ziwei Xiao <ziweixiao@google.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Reviewed-by: Shailend Chand <shailend@google.com>
Reviewed-by: Praveen Kaligineedi <pkaligineedi@google.com>
Link: https://lore.kernel.org/r/20240612001654.923887-1-ziweixiao@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>