x86/vdso: Fix latent bug in vclock_pages calculation

The vclock pages are *after* the non-vclock pages. Currently there are both
two vclock and two non-vclock pages so the existing logic works by
accident.  As soon as the number of pages changes it will break however.
This will be the case with the introduction of the generic vDSO data
storage.

Use a macro to keep the calculation understandable and in sync between
the linker script and mapping code.

Fixes: e93d2521 ("x86/vdso: Split virtual clock pages into dedicated mapping")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250204-vdso-store-rng-v3-1-13a4669dfc8c@linutronix.de