x86/vdso: Place vdso_data at beginning of vvar page

The offset of the vdso_data only has historic reasons, as back then
other vvars also existed and offset 0 was already used.
(See commit 8c49d9a7 ("x86-64: Clean up vdso/kernel shared variables"))
Over time most other vvars got removed and offset 0 is free again.

Moving vdso_data to the beginning of the vvar page aligns x86 with other
architectures and opens up the way for the removal of the custom x86
vvar machinery.

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-v1-12-b64f0842d512@linutronix.de