time/sched_clock: Use ACCESS_PRIVATE() to evaluate hrtimer::function

This dereference of sched_clock_timer::function was missed when the
hrtimer callback function pointer was marked private.

Fixes: 04257da ("hrtimers: Make callback function pointer private")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/875x95jw7q.ffs@tglx
Closes: https://lore.kernel.org/oe-kbuild-all/202601131713.KsxhXQ0M-lkp@intel.com/

Author: Thomas Gleixner

kernel/time/sched_clock.c

@@ -215,7 +215,7 @@ void sched_clock_register(u64 (*read)(void), int bits, unsigned long rate)
 
 	update_clock_read_data(&rd);
 
-	if (sched_clock_timer.function != NULL) {
+	if (ACCESS_PRIVATE(&sched_clock_timer, function) != NULL) {
 		/* update timeout for clock wrap */
 		hrtimer_start(&sched_clock_timer, cd.wrap_kt,
 			      HRTIMER_MODE_REL_HARD);