igb: Improve performance and reduce size of igb_tx_map

This change is meant to both improve the performance and reduce the size of
igb_tx_map.  To do this I have expanded the work done in the main loop by
pushing first into tx_buffer.  This allows us to pull in the dma_mapping_error
check, the tx_buffer value assignment, and the initial DMA value assignment to
the Tx descriptor.  The net result is that the function reduces in size by a
little over a 100 bytes and is about 1% or 2% faster.

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>
Signed-off-by: David S. Miller <davem@davemloft.net>