e1000 uses the same metadata struct for Rx and Tx.  But Tx and Rx have
different requirements.

For Rx, we only need to store a buffer and a DMA address.

Follow-up patch will remove skb for Rx, bringing rx_buffer_info down
to 16 bytes on x86_64.

[ buffer_info is 48 bytes ]
Signed-off-by: Florian Westphal <fw@strlen.de>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>