TX SKBs will have their buffers DMA mapped with the device. Each buffer
will have at least one TX descriptor associated. Each SKB will also have
a metadata descriptor.

Each TX queue maintains an array of `gve_tx_pending_packet_dqo` objects.
Every TX SKB will have an associated pending_packet object. A TX SKB's
descriptors will use its pending_packet's index as the completion tag,
which will be returned on the TX completion queue.

The device implements a "flow-miss model". Most packets will simply
receive a packet completion. The flow-miss system may choose to process
a packet based on its contents. A TX packet which experiences a flow
miss would receive a miss completion followed by a later reinjection
completion. The miss-completion is received when the packet starts to be
processed by the flow-miss system and the reinjection completion is
received when the flow-miss system completes processing the packet and
sends it on the wire.

Notable mentions:

- Buffers may be freed after receiving the miss-completion, but in order
  to avoid packet reordering, we do not complete the SKB until receiving
  the reinjection completion.

- The driver must robustly handle the unlikely scenario where a miss
  completion does not have an associated reinjection completion. This is
  accomplished by maintaining a list of packets which have a pending
  reinjection completion. After a short timeout (5 seconds), the
  SKB and buffers are released and the pending_packet is moved to a
  second list which has a longer timeout (60 seconds), where the
  pending_packet will not be reused. When the longer timeout elapses,
  the driver may assume the reinjection completion would never be
  received and the pending_packet may be reused.

- Completion handling is triggered by an interrupt and is done in the
  NAPI poll function. Because the TX path and completion exist in
  different threading contexts they maintain their own lists for free
  pending_packet objects. The TX path uses a lock-free approach to steal
  the list from the completion path.

- Both the TSO context and general context descriptors have metadata
  bytes. The device requires that if multiple descriptors contain the
  same field, each descriptor must have the same value set for that
  field.
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>