igbvf: add lbtx_packets and lbtx_bytes to ethtool statistics

igbvf: add lbtx_packets and lbtx_bytes to ethtool statistics

Currently ethtool shows lbrx_packets and lbrx_bytes (Good RX
Packets/Octets loopback Count), but doesn't show the TX-side equivalents
(lbtx_packets and lbtx_bytes). Add visibility of those missing
statistics by adding them to ethtool statistics.

In addition, the order of lbrx_bytes and lbrx_packets is not consistent
with non-loopback statistics (rx_packets, rx_bytes). Therefore,
align the order by swapping positions of lbrx_bytes and lbrx_packets.

Tested on Intel Corporation I350 Gigabit Network Connection.

Before:
  # ethtool -S ens5 | grep -E "x_(bytes|packets)"
       rx_packets: 135
       tx_packets: 106
       rx_bytes: 16010
       tx_bytes: 12451
       lbrx_bytes: 1148
       lbrx_packets: 12

After:
  # ethtool -S ens5 | grep -E "x_(bytes|packets)"
       rx_packets: 748
       tx_packets: 304
       rx_bytes: 81513
       tx_bytes: 33698
       lbrx_packets: 97
       lbtx_packets: 109
       lbrx_bytes: 12090
       lbtx_bytes: 12401

Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Kohei Enju <enjuk@amazon.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>