posix_timers: Fix pre-condition to stop the tick on full dynticks

The test that checks if a CPU can stop its tick from posix CPU
timers angle was mistakenly inverted.

What we want is to prevent the tick from being stopped as long
as the current CPU's task runs a posix CPU timer.

Fix this.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

kernel/posix-cpu-timers.c

@@ -673,12 +673,12 @@ static void posix_cpu_timer_kick_nohz(void)
 bool posix_cpu_timers_can_stop_tick(struct task_struct *tsk)
 {
 	if (!task_cputime_zero(&tsk->cputime_expires))
-		return true;
+		return false;
 
 	if (tsk->signal->cputimer.running)
-		return true;
+		return false;
 
-	return false;
+	return true;
 }
 #else
 static inline void posix_cpu_timer_kick_nohz(void) { }