Igbe: Newer kernels seem to allow TSO headers and packet data to be in one desc

Igbe: Newer kernels seem to allow TSO headers and packet data to be in one desc

Implement some code we used to panic on as it actually does happen with the
e1000 driver in Linux 3.3+. We used to assume that a TSO header would never
be part of a larger payload, however it appears as though it now can be.