nds32: Fix vDSO clock_getres()

clock_getres in the vDSO library has to preserve the same behaviour
of posix_get_hrtimer_res().

In particular, posix_get_hrtimer_res() does:
    sec = 0;
    ns = hrtimer_resolution;
and hrtimer_resolution depends on the enablement of the high
resolution timers that can happen either at compile or at run time.

Fix the nds32 vdso implementation of clock_getres keeping a copy of
hrtimer_resolution in vdso data and using that directly.

Cc: Greentime Hu <green.hu@gmail.com>
Cc: Vincent Chen <deanbo422@gmail.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>

arch/nds32/include/asm/vdso_datapage.h

@@ -20,6 +20,7 @@ struct vdso_data {
 	u32 xtime_clock_sec;	/* CLOCK_REALTIME - seconds */
 	u32 cs_mult;		/* clocksource multiplier */
 	u32 cs_shift;		/* Cycle to nanosecond divisor (power of two) */
+	u32 hrtimer_res;	/* hrtimer resolution */
 
 	u64 cs_cycle_last;	/* last cycle value */
 	u64 cs_mask;		/* clocksource mask */

arch/nds32/kernel/vdso.c

@@ -220,6 +220,7 @@ void update_vsyscall(struct timekeeper *tk)
 	vdso_data->xtime_coarse_sec = tk->xtime_sec;
 	vdso_data->xtime_coarse_nsec = tk->tkr_mono.xtime_nsec >>
 	    tk->tkr_mono.shift;
+	vdso_data->hrtimer_res = hrtimer_resolution;
 	vdso_write_end(vdso_data);
 }
 

arch/nds32/kernel/vdso/gettimeofday.c

@@ -208,6 +208,8 @@ static notrace int clock_getres_fallback(clockid_t _clk_id,
 
 notrace int __vdso_clock_getres(clockid_t clk_id, struct timespec *res)
 {
+	struct vdso_data *vdata = __get_datapage();
+
 	if (res == NULL)
 		return 0;
 	switch (clk_id) {
@@ -215,7 +217,7 @@ notrace int __vdso_clock_getres(clockid_t clk_id, struct timespec *res)
 	case CLOCK_MONOTONIC:
 	case CLOCK_MONOTONIC_RAW:
 		res->tv_sec = 0;
-		res->tv_nsec = CLOCK_REALTIME_RES;
+		res->tv_nsec = vdata->hrtimer_res;
 		break;
 	case CLOCK_REALTIME_COARSE:
 	case CLOCK_MONOTONIC_COARSE: