gro: Restore correct value to gso_size

Since everybody has been focusing on baremetal GRO performance
no one noticed when I added a bug that zapped gso_size for all
GRO packets.  This only gets picked up when you forward the skb
out of an interface.

Thanks to Mark Wagner for noticing this bug when testing kvm.

Reported-by: Mark Wagner <mwagner@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>