ptp: vmclock: Use hw_cycles from snapshot for precise TSC pairing

ptp: vmclock: Use hw_cycles from snapshot for precise TSC pairing

When the system clocksource is kvmclock or Hyper-V (not the TSC directly),
vmclock_get_crosststamp() falls through to a separate get_cycles() call,
losing the atomic pairing between the system time snapshot and the TSC
reading.

Now that ktime_get_snapshot_id() populates hw_cycles with the underlying
TSC value for derived clocksources, use it when available.  This gives a
perfect (system_time, tsc) pairing for the device time calculation.

The SUPPORT_KVMCLOCK wrapper is still needed to convert the TSC into
kvmclock nanoseconds for system_counter->cycles, because otherwise
get_device_system_crosststamp() can't interpret the result against the
system clock.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Assisted-by: Kiro:claude-opus-4.6-1m
Link: https://patch.msgid.link/20260604095755.64849-4-dwmw2@infradead.org