x86/vdso: Fix function reference in comment

Replace the reference to the non-existent function arch_vdso_cycles_valid()
by the proper function arch_vdso_cycles_ok().

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20240701-vdso-cleanup-v1-3-36eb64e7ece2@linutronix.de