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>