igb: split buffer_info into tx_buffer_info and rx_buffer_info

In order to be able to improve the performance of the TX path it has been
necessary to add addition info to the tx_buffer_info structure.  However a
side effect is that the structure has gotten larger and this in turn has
also increased the size of the RX buffer info structure.  In order to avoid
this in the future I am splitting the single buffer_info structure into two
separate ones and instead I will join them by making the buffer_info
pointer in the ring a union of the two.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>