pps: Convert to ktime_get_snapshot_id()

pps: Convert to ktime_get_snapshot_id()

ktime_get_snapshot() resolves to ktime_get_snapshot_id(CLOCK_REALTIME).

Make it obvious in the code and convert the readout to use the
snapshot::systime and monoraw fields instead of snapshot::real and raw,
which aregoing away.

Similar to the PPS generators, avoid the more expensive snapshot when
CONFIG_NTP_PPS is disabled.

No functional change intended.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Arthur Kiyanovski <akiyano@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20260529195557.123410250@kernel.org