gve: Implement ndo_hwtstamp_get/set for RX timestamping

gve: Implement ndo_hwtstamp_get/set for RX timestamping

Implement ndo_hwtstamp_get/set to enable hardware RX timestamping,
providing support for SIOC[SG]HWTSTAMP IOCTLs. Included with this support
is the small change necessary to read the rx timestamp out of the rx
descriptor, now that timestamps start being enabled. The gve clock is
only used for hardware timestamps, so started when timestamps are
requested and stopped when not needed.

This version only supports RX hardware timestamping with the rx filter
HWTSTAMP_FILTER_ALL. If the user attempts to configure a more
restrictive filter, the filter will be set to HWTSTAMP_FILTER_ALL in the
returned structure.

Signed-off-by: John Fraker <jfraker@google.com>
Signed-off-by: Ziwei Xiao <ziweixiao@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
Link: https://patch.msgid.link/20250614000754.164827-8-hramamurthy@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>