gro: Fix bogus gso_size on the first fraglist entry

commit 622e0ca1 upstream.

When GRO produces fraglist entries, and the resulting skb hits
an interface that is incapable of TSO but capable of FRAGLIST,
we end up producing a bogus packet with gso_size non-zero.

This was reported in the field with older versions of KVM that
did not set the TSO bits on tuntap.

This patch fixes that.
Reported-by: Igor Zhang <yugzhang@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

net/core/skbuff.c

@@ -2730,6 +2730,7 @@ int skb_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 	*NAPI_GRO_CB(nskb) = *NAPI_GRO_CB(p);
 	skb_shinfo(nskb)->frag_list = p;
 	skb_shinfo(nskb)->gso_size = pinfo->gso_size;
+	pinfo->gso_size = 0;
 	skb_header_release(p);
 	nskb->prev = p;