net: add hwtstamping helpers for stackable net devices

The stackable net devices with hwtstamping support (vlan, macvlan,
bonding) only pass the hwtstamping ops to the lower (real) device.

These drivers are the first that need to be converted to the new
timestamping API, because if they aren't prepared to handle that,
then no real device driver cannot be converted to the new API either.

After studying what vlan_dev_ioctl(), macvlan_eth_ioctl() and
bond_eth_ioctl() have in common, here we propose two generic
implementations of ndo_hwtstamp_get() and ndo_hwtstamp_set() which
can be called by those 3 drivers, with "dev" being their lower device.

These helpers cover both cases, when the lower driver is converted to
the new API or unconverted.

We need some hacks in case of an unconverted driver, namely to stuff
some pointers in struct kernel_hwtstamp_config which shouldn't have
been there (since the new API isn't supposed to need it). These will
be removed when all drivers will have been converted to the new API.

Signed-off-by: Maxim Georgiev <glipus@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20230801142824.1772134-3-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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