PM / Sleep: Make [un]lock_system_sleep() generic

The [un]lock_system_sleep() APIs were originally introduced to mutually
exclude memory hotplug and hibernation.

Directly using mutex_lock(&pm_mutex) to achieve mutual exclusion with
suspend or hibernation code can lead to freezing failures. However, the
APIs [un]lock_system_sleep() can be safely used to achieve the same,
without causing freezing failures.

So, since it would be beneficial to modify all the existing users of
mutex_lock(&pm_mutex) (in all parts of the kernel), so that they use these
safe APIs intead, make these APIs generic by removing the restriction that
they work only when CONFIG_HIBERNATE_CALLBACKS is set. Moreover, that
restriction didn't buy us anything anyway.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

include/linux/suspend.h

@@ -332,6 +332,8 @@ static inline bool system_entering_hibernation(void) { return false; }
 #define PM_RESTORE_PREPARE	0x0005 /* Going to restore a saved image */
 #define PM_POST_RESTORE		0x0006 /* Restore failed */
 
+extern struct mutex pm_mutex;
+
 #ifdef CONFIG_PM_SLEEP
 void save_processor_state(void);
 void restore_processor_state(void);
@@ -352,6 +354,19 @@ extern bool events_check_enabled;
 extern bool pm_wakeup_pending(void);
 extern bool pm_get_wakeup_count(unsigned int *count);
 extern bool pm_save_wakeup_count(unsigned int count);
+
+static inline void lock_system_sleep(void)
+{
+	freezer_do_not_count();
+	mutex_lock(&pm_mutex);
+}
+
+static inline void unlock_system_sleep(void)
+{
+	mutex_unlock(&pm_mutex);
+	freezer_count();
+}
+
 #else /* !CONFIG_PM_SLEEP */
 
 static inline int register_pm_notifier(struct notifier_block *nb)
@@ -367,30 +382,11 @@ static inline int unregister_pm_notifier(struct notifier_block *nb)
 #define pm_notifier(fn, pri)	do { (void)(fn); } while (0)
 
 static inline bool pm_wakeup_pending(void) { return false; }
-#endif /* !CONFIG_PM_SLEEP */
-
-extern struct mutex pm_mutex;
 
-#ifndef CONFIG_HIBERNATE_CALLBACKS
 static inline void lock_system_sleep(void) {}
 static inline void unlock_system_sleep(void) {}
 
-#else
-
-/* Let some subsystems like memory hotadd exclude hibernation */
-
-static inline void lock_system_sleep(void)
-{
-	freezer_do_not_count();
-	mutex_lock(&pm_mutex);
-}
-
-static inline void unlock_system_sleep(void)
-{
-	mutex_unlock(&pm_mutex);
-	freezer_count();
-}
-#endif
+#endif /* !CONFIG_PM_SLEEP */
 
 #ifdef CONFIG_ARCH_SAVE_PAGE_KEYS
 /*