cpuidle: Fix panic in CPU off-lining with no idle driver

Fix a NULL pointer dereference panic in cpuidle_play_dead() during
CPU off-lining when no cpuidle driver is registered.  A cpuidle
driver may be registered at boot-time based on CPU type.  This patch
allows an off-lined CPU to enter HLT-based idle in this condition.
Signed-off-by: Toshi Kani <toshi.kani@hp.com>
Cc: Boris Ostrovsky <boris.ostrovsky@amd.com>
Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Tested-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/cpuidle/cpuidle.c

@@ -74,7 +74,7 @@ static cpuidle_enter_t cpuidle_enter_ops;
 /**
  * cpuidle_play_dead - cpu off-lining
  *
- * Only returns in case of an error
+ * Returns in case of an error or no driver
  */
 int cpuidle_play_dead(void)
 {
@@ -83,6 +83,9 @@ int cpuidle_play_dead(void)
 	int i, dead_state = -1;
 	int power_usage = -1;
 
+	if (!drv)
+		return -ENODEV;
+
 	/* Find lowest-power state that supports long-term idle */
 	for (i = CPUIDLE_DRIVER_STATE_START; i < drv->state_count; i++) {
 		struct cpuidle_state *s = &drv->states[i];