posix-cpu-timers: Make set_process_cpu_timer() more robust

Because the return value of cpu_timer_sample_group() is not checked,
compilers and static checkers can legitimately warn about a potential use
of the uninitialized variable 'now'. This is not a runtime issue as all call
sites hand in valid clock ids.

Also cpu_timer_sample_group() is invoked unconditionally even when the
result is not used because *oldval is NULL.

Make the invocation conditional and check the return value.

[ tglx: Massage changelog ]
Signed-off-by: Max R. P. Grossmann <m@max.pm>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: john.stultz@linaro.org
Link: https://lkml.kernel.org/r/20180108190157.10048-1-m@max.pm

kernel/time/posix-cpu-timers.c

@@ -1189,9 +1189,8 @@ void set_process_cpu_timer(struct task_struct *tsk, unsigned int clock_idx,
 	u64 now;
 
 	WARN_ON_ONCE(clock_idx == CPUCLOCK_SCHED);
-	cpu_timer_sample_group(clock_idx, tsk, &now);
 
-	if (oldval) {
+	if (oldval && cpu_timer_sample_group(clock_idx, tsk, &now) != -EINVAL) {
 		/*
 		 * We are setting itimer. The *oldval is absolute and we update
 		 * it to be relative, *newval argument is relative and we update