virtio_net: Set correct gso->hdr_len

Through a bug in the tun driver, I noticed that virtio_net is
producing bogus hdr_len values.  In particular, it only includes
the IP header in the linear area, and excludes the entire TCP
header.  This causes the TCP header to be copied twice for each
packet.  (The bug omitted the second copy :)

This patch corrects this.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>