netdev: clarify GSO vs csum in qstats

netdev: clarify GSO vs csum in qstats

Could be just me, but I had to pause and double check that the Tx csum
counter in qstat should include GSO'd packets. GSO pretty much implies
csum so one could possibly interpret the csum counter as pure csum offload.

But the counters are based on virtio:

  [tx_needs_csum]
      The number of packets which require checksum calculation by the device.

  [rx_needs_csum]
      The number of packets with VIRTIO_NET_HDR_F_NEEDS_CSUM.

and VIRTIO_NET_HDR_F_NEEDS_CSUM gets set on GSO packets virtio sends.

Clarify this in the spec to avoid any confusion.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250214224601.2271201-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>