x86/vdso: Access vdso data without vvar.h

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

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-v1-18-b64f0842d512@linutronix.de