net-gro: Fix GRO flush when receiving a GSO packet.

Currently we may merge incorrectly a received GSO packet
or a packet with frag_list into a packet sitting in the
gro_hash list. skb_segment() may crash case because
the assumptions on the skb layout are not met.
The correct behaviour would be to flush the packet in the
gro_hash list and send the received GSO packet directly
afterwards. Commit d61d072e ("net-gro: avoid reorders")
sets NAPI_GRO_CB(skb)->flush in this case, but this is not
checked before merging. This patch makes sure to check this
flag and to not merge in that case.

Fixes: d61d072e ("net-gro: avoid reorders")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -3801,7 +3801,7 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
 	unsigned int delta_truesize;
 	struct sk_buff *lp;
 
-	if (unlikely(p->len + len >= 65536))
+	if (unlikely(p->len + len >= 65536 || NAPI_GRO_CB(skb)->flush))
 		return -E2BIG;
 
 	lp = NAPI_GRO_CB(p)->last;