igb: move gso_segs into buffer_info structure

This change moves gso_segs into the buffer_info structure to avoid
a possible cache line miss in clean_tx_irq.

Signed-off-by: Nicholas Nunley <nicholasx.d.nunley@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>