gro: fix aggregation for skb using frag_list

In commit 8a29111c ("net: gro: allow to build full sized skb")
I added a regression for linear skb that traditionally force GRO
to use the frag_list fallback.

Erez Shitrit found that at most two segments were aggregated and
the "if (skb_gro_len(p) != pinfo->gso_size)" test was failing.

This is because pinfo at this spot still points to the last skb in the
chain, instead of the first one, where we find the correct gso_size
information.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Fixes: 8a29111c ("net: gro: allow to build full sized skb")
Reported-by: NErez Shitrit <erezsh@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -3152,6 +3152,9 @@ int skb_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 		NAPI_GRO_CB(skb)->free = NAPI_GRO_FREE_STOLEN_HEAD;
 		goto done;
 	}
+	/* switch back to head shinfo */
+	pinfo = skb_shinfo(p);
+
 	if (pinfo->frag_list)
 		goto merge;
 	if (skb_gro_len(p) != pinfo->gso_size)