gve: Add Rx HWTS metadata to AF_XDP ZC mode

gve: Add Rx HWTS metadata to AF_XDP ZC mode

By overlaying the struct gve_xdp_buff on top of the struct xdp_buff_xsk
that AF_XDP utilizes, the driver records the 32 bit timestamp via the
completion descriptor and the cached 64 bit NIC timestamp via gve_priv.

The driver's implementation of xmo_rx_timestamp extends the timestamp to
the full and up to date 64 bit timestamp and returns it to the user.

gve_rx_xsk_dqo is modified to accept a pointer to the completion
descriptor and no longer takes a buf_len explicitly as it can be pulled
out of the descriptor.

With this patch gve now supports bpf_xdp_metadata_rx_timestamp.

Signed-off-by: Tim Hostetler <thostet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Signed-off-by: Joshua Washington <joshwash@google.com>
Link: https://patch.msgid.link/20251114211146.292068-5-joshwash@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>