ravb: Fix potential use-after-free in ravb_rx_gbeth()

The skb is delivered to napi_gro_receive() which may free it, after calling this,
dereferencing skb may trigger use-after-free.

Fixes: 1c59eb67

 ("ravb: Fillup ravb_rx_gbeth() stub")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20221203092941.10880-1-yuehaibing@huawei.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>