igb: Support using build_skb in the case that jumbo frames are disabled

This change makes it so that we can enable the use of build_skb for cases
where jumbo frames are disabled.  The advantage to this is that we do not
have to perform a memcpy to populate the header and as a result we see a
significant performance improvement.

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>