gve: fix frag_list chaining

gve_rx_append_frags() is able to build skbs chained with frag_list,
like GRO engine.

Problem is that shinfo->frag_list should only be used
for the head of the chain.

All other links should use skb->next pointer.

Otherwise, built skbs are not valid and can cause crashes.

Equivalent code in GRO (skb_gro_receive()) is:

    if (NAPI_GRO_CB(p)->last == p)
        skb_shinfo(p)->frag_list = skb;
    else
        NAPI_GRO_CB(p)->last->next = skb;
    NAPI_GRO_CB(p)->last = skb;

Fixes: 9b8dd5e ("gve: DQO: Add RX path")
Signed-off-by: Eric Dumazet <[email protected]>
Cc: Bailey Forrest <[email protected]>
Cc: Willem de Bruijn <[email protected]>
Cc: Catherine Sullivan <[email protected]>
Reviewed-by: David Ahern <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: edumazet

drivers/net/ethernet/google/gve/gve_rx_dqo.c

@@ -570,7 +570,10 @@ static int gve_rx_append_frags(struct napi_struct *napi,
 		if (!skb)
 			return -1;
 
-		skb_shinfo(rx->ctx.skb_tail)->frag_list = skb;
+		if (rx->ctx.skb_tail == rx->ctx.skb_head)
+			skb_shinfo(rx->ctx.skb_head)->frag_list = skb;
+		else
+			rx->ctx.skb_tail->next = skb;
 		rx->ctx.skb_tail = skb;
 		num_frags = 0;
 	}