timekeeping: Rename fast_tk_get_delta_ns() to __timekeeping_get_ns()

Rename fast_tk_get_delta_ns() to __timekeeping_get_ns() to prepare for its
reuse as a general timekeeping helper function.

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/20240325064023.2997-11-adrian.hunter@intel.com