tilegx: fix some issues in the SW TSO support

This change correctly computes the header length and data length in
the fragments to avoid a bug where we would end up with extremely
slow performance.  Also adopt use of skb_frag_size() accessor.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Cc: stable@vger.kernel.org [v3.6]
Signed-off-by: David S. Miller <davem@davemloft.net>