cpu-timers: Return correct previous timer reload value

According POSIX we need to correctly set old timer it_interval value when
user request that in timer_settime().  Tested using below program.

 #include <sys/time.h>
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>
 #include <unistd.h>
 #include <assert.h>

 int main(void)
 {
	struct sigaction act;
	struct sigevent evt = { };
	timer_t tid;
	struct itimerspec spec, u_spec, k_spec;

	evt.sigev_notify = SIGEV_SIGNAL;
	evt.sigev_signo = SIGPROF;
	assert(timer_create(CLOCK_PROCESS_CPUTIME_ID, &evt,  &tid) == 0);

	spec.it_value.tv_sec = 1;
	spec.it_value.tv_nsec = 2;
	spec.it_interval.tv_sec = 3;
	spec.it_interval.tv_nsec = 4;
	u_spec = spec;
	assert(timer_settime(tid, 0, &spec,  NULL) == 0);

	spec.it_value.tv_sec = 5;
	spec.it_value.tv_nsec = 6;
	spec.it_interval.tv_sec = 7;
	spec.it_interval.tv_nsec = 8;
	assert(timer_settime(tid, 0, &spec,  &k_spec) == 0);

 #define PRT(val) printf(#val ":\t%d/%d\n", (int) u_spec.val, (int) k_spec.val)
	PRT(it_value.tv_sec);
	PRT(it_value.tv_nsec);
	PRT(it_interval.tv_sec);
	PRT(it_interval.tv_nsec);

	return 0;
 }

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>