posix-timers: Correct sanity check in posix_cpu_nsleep

CPUCLOCK_PID(which_clock) is a pid value from userspace so compare it
against task_pid_vnr, not current->pid.  As task_pid_vnr is in the tasks
pid value in the tasks pid namespace, and current->pid is in the
initial pid namespace.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

kernel/time/posix-cpu-timers.c

@@ -1312,7 +1312,7 @@ static int posix_cpu_nsleep(const clockid_t which_clock, int flags,
 	 */
 	if (CPUCLOCK_PERTHREAD(which_clock) &&
 	    (CPUCLOCK_PID(which_clock) == 0 ||
-	     CPUCLOCK_PID(which_clock) == current->pid))
+	     CPUCLOCK_PID(which_clock) == task_pid_vnr(current)))
 		return -EINVAL;
 
 	error = do_cpu_nanosleep(which_clock, flags, rqtp, &it);