vdso: Consolidate vdso_calc_delta()

Consolidate vdso_calc_delta(), in preparation for further simplification.

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/20240325064023.2997-2-adrian.hunter@intel.com