igb: Lock buffer size at 2K even on systems with larger pages

This change locks us in at 2K buffers even on a system that supports larger
frames.  The reason for this change is to make better use of pages and to
reduce the overall truesize of frames generated by igb.

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>