[NET]: Fix logical error in skb_gso_ok

The test in skb_gso_ok is backwards.  Noticed by Michael Chan
<mchan@broadcom.com>.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>