timekeeping: Prepare timekeeping_cycles_to_ns() for overflow safety

Open code clocksource_delta() in timekeeping_cycles_to_ns() so that
overflow safety can be added efficiently.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240325064023.2997-17-adrian.hunter@intel.com