posix-cpu-timers: Simplify posix_cpu_timer_set()

Avoid the late sighand lock/unlock dance when a timer is not armed to
enforce reevaluation of the timer base so that the process wide CPU timer
sampling can be disabled.

Do it right at the point where the arming decision is made which already
has sighand locked.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>