LoongArch: vDSO: Use vdso/datapage.h to access vDSO data

LoongArch: vDSO: Use vdso/datapage.h to access vDSO data

vdso/datapage.h provides symbols and functions to ease the access to
shared vDSO data from both the kernel and the vDSO.
Make use of it to simplify the current code and also prepare for further
changes unifying the vDSO data storage between architectures.

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-v1-8-b64f0842d512@linutronix.de