gso: Stop fraglists from escaping

As it stands skb fraglists can get past the check in dev_queue_xmit
if the skb is marked as GSO.  In particular, if the packet doesn't
have the proper checksums for GSO, but can otherwise be handled by
the underlying device, we will not perform the fraglist check on it
at all.

If the underlying device cannot handle fraglists, then this will
break.

The fix is as simple as moving the fraglist check from the device
check into skb_gso_ok.

This has caused crashes with Xen when used together with GRO which
can generate GSO packets with fraglists.

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