posix-timers: Store k_clock pointer in k_itimer

Having the k_clock pointer in the k_itimer struct avoids the lookup in
several code pathes and makes the next steps of unification of the hrtimer
and alarmtimer based posix timers simpler.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/20170530211656.641222072@linutronix.de

kernel/time/posix-cpu-timers.c

@@ -324,6 +324,8 @@ static int posix_cpu_timer_create(struct k_itimer *new_timer)
 	if (CPUCLOCK_WHICH(new_timer->it_clock) >= CPUCLOCK_MAX)
 		return -EINVAL;
 
+	new_timer->kclock = &clock_posix_cpu;
+
 	INIT_LIST_HEAD(&new_timer->it.cpu.entry);
 
 	rcu_read_lock();