igb: Revert support for build_skb in igb

This patch actually reverts:
igb: Support using build_skb in the case that jumbo frames are disabled

The reason for reverting this patch is that it can lead to data corruption.
The following flow was pointed out by Ben Hutchings:

1. skb is forwarded to another device
2. Packet headers are modified and it's put into a queue
3. Second packet is received into the other half of this page
4. Page cannot be reused, so is DMA-unmapped
5. The DMA mapping was non-coherent, so unmap copies or invalidates
cache

The headers added in step 2 get trashed in step 5.

Reported-by: Ben Hutchings <bhutchings@solarflare.com>
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>