gve: Add DQO fields for core data structures

- Add new DQO datapath structures:
  - `gve_rx_buf_queue_dqo`
  - `gve_rx_compl_queue_dqo`
  - `gve_rx_buf_state_dqo`
  - `gve_tx_desc_dqo`
  - `gve_tx_pending_packet_dqo`

- Incorporate these into the existing ring data structures:
  - `gve_rx_ring`
  - `gve_tx_ring`

Noteworthy mentions:

- `gve_rx_buf_state` represents an RX buffer which was posted to HW.
  Each RX queue has an array of these objects and the index into the
  array is used as the buffer_id when posted to HW.

- `gve_tx_pending_packet_dqo` is treated similarly for TX queues. The
  completion_tag is the index into the array.

- These two structures have links for linked lists which are represented
  by 16b indexes into a contiguous array of these structures.
  This reduces memory footprint compared to 64b pointers.

- We use unions for the writeable datapath structures to reduce cache
  footprint. GQI specific members will renamed like DQO members in a
  future patch.

Signed-off-by: Bailey Forrest <bcf@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Catherine Sullivan <csully@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>