PM / sleep: Fix test_suspend= command line option

After commit d431cbc53cb7 (PM / sleep: Simplify sleep states sysfs
interface code) the pm_states[] array is not populated initially,
which causes setup_test_suspend() to always fail and the suspend
testing during boot doesn't work any more.

Fix the problem by using pm_labels[] instead of pm_states[] in
setup_test_suspend() and storing a pointer to the label of the
sleep state to test rather than the number representing it,
because the connection between the state numbers and labels is
only established by suspend_set_ops().

Fixes: d431cbc53cb7 (PM / sleep: Simplify sleep states sysfs interface code)
Reported-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
diff --git a/kernel/power/suspend.c b/kernel/power/suspend.c
index 6dadb25..18c6219 100644
--- a/kernel/power/suspend.c
+++ b/kernel/power/suspend.c
@@ -31,7 +31,7 @@
 
 #include "power.h"
 
-static const char *pm_labels[] = { "mem", "standby", "freeze", };
+const char *pm_labels[] = { "mem", "standby", "freeze", NULL };
 const char *pm_states[PM_SUSPEND_MAX];
 
 static const struct platform_suspend_ops *suspend_ops;