gve: Add stats for gve.

gve: Add stats for gve.

Sample output of "ethtool -S <interface-name>" with 1 RX queue and 1 TX
queue:
NIC statistics:
     rx_packets: 1039
     tx_packets: 37
     rx_bytes: 822071
     tx_bytes: 4100
     rx_dropped: 0
     tx_dropped: 0
     tx_timeouts: 0
     rx_skb_alloc_fail: 0
     rx_buf_alloc_fail: 0
     rx_desc_err_dropped_pkt: 0
     interface_up_cnt: 1
     interface_down_cnt: 0
     reset_cnt: 0
     page_alloc_fail: 0
     dma_mapping_error: 0
     rx_posted_desc[0]: 1365
     rx_completed_desc[0]: 341
     rx_bytes[0]: 215094
     rx_dropped_pkt[0]: 0
     rx_copybreak_pkt[0]: 3
     rx_copied_pkt[0]: 3
     tx_posted_desc[0]: 6
     tx_completed_desc[0]: 6
     tx_bytes[0]: 420
     tx_wake[0]: 0
     tx_stop[0]: 0
     tx_event_counter[0]: 6
     adminq_prod_cnt: 34
     adminq_cmd_fail: 0
     adminq_timeouts: 0
     adminq_describe_device_cnt: 1
     adminq_cfg_device_resources_cnt: 1
     adminq_register_page_list_cnt: 16
     adminq_unregister_page_list_cnt: 0
     adminq_create_tx_queue_cnt: 8
     adminq_create_rx_queue_cnt: 8
     adminq_destroy_tx_queue_cnt: 0
     adminq_destroy_rx_queue_cnt: 0
     adminq_dcfg_device_resources_cnt: 0
     adminq_set_driver_parameter_cnt: 0

Reviewed-by: Yangchun Fu <yangchun@google.com>
Signed-off-by: Kuo Zhao <kuozhao@google.com>
Signed-off-by: David Awogbemila <awogbemila@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>