tilegx net driver: handle payload data not in frags

The original driver implementation assumed that for TSO, all the
payload data would be in the frags.  This isn't always true; change
the driver to support payload data at skb->data between
"skb_transport_offset(skb) + tcp_hdrlen(skb)" and "skb->hdr_len",
followed by the data in the frags.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>