vdso/datapage: Define vdso_clock to prepare for multiple PTP clocks

Multiple PTP clocks, which are independent of timekeeping, are required for
systems, which utilize PTP for synchronizing e.g. automation systems
independent of clock TAI.

PTP clocks are slow to access, but applications require fast access to the
relevant time similar to the regular timekeeping relevant clocks.

To prepare for that the VDSO data representation must be reworked. For
transition to the new structure of the vdso, add a define which maps
vdso_clock to vdso_data. This will be removed when all users are updated
step by step.

No functional change.

Signed-off-by: Anna-Maria Behnsen <[email protected]>
Signed-off-by: Nam Cao <[email protected]>
Signed-off-by: Thomas Weißschuh <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/all/[email protected]

Author: anna-marialx

include/vdso/datapage.h

@@ -129,6 +129,8 @@ struct vdso_time_data {
 	struct arch_vdso_time_data arch_data;
 } ____cacheline_aligned;
 
+#define vdso_clock vdso_time_data
+
 /**
  * struct vdso_rng_data - vdso RNG state information
  * @generation:	counter representing the number of RNG reseeds