x86/vdso: Access timens vdso data without vvar.h

The vdso_data is at the start of the timens page.
Make use of this invariant to remove the usage of vvar.h.
This also matches the logic for the pvclock and hvclock pages.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241010-vdso-generic-base-v1-15-b64f0842d512@linutronix.de