gve: Prepare bpf_xdp_metadata_rx_timestamp support

gve: Prepare bpf_xdp_metadata_rx_timestamp support

Support populating XDP RX metadata with hardware RX timestamps. This
patch utilizes the same underlying logic to calculate hardware
timestamps as the regular RX path.

xdp_metadata_ops is registered with the net_device in a future patch.

gve_rx_calculate_hwtstamp was pulled out so as to not duplicate logic
between gve_xdp_rx_timestamp and gve_rx_hwtstamp.

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/20251114211146.292068-4-joshwash@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>