PM / Sleep: Use the freezer_count() functions in [un]lock_system_sleep() APIs

Now that freezer_count() and freezer_do_not_count() don't have the restriction
that they are effective only when called by userspace processes, use
them in lock_system_sleep() and unlock_system_sleep() instead of open-coding
their parts.
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

include/linux/suspend.h

@@ -6,6 +6,7 @@
 #include <linux/init.h>
 #include <linux/pm.h>
 #include <linux/mm.h>
+#include <linux/freezer.h>
 #include <asm/errno.h>
 
 #ifdef CONFIG_VT
@@ -380,16 +381,14 @@ static inline void unlock_system_sleep(void) {}
 
 static inline void lock_system_sleep(void)
 {
-	/* simplified freezer_do_not_count() */
-	current->flags |= PF_FREEZER_SKIP;
+	freezer_do_not_count();
 	mutex_lock(&pm_mutex);
 }
 
 static inline void unlock_system_sleep(void)
 {
 	mutex_unlock(&pm_mutex);
-	/* simplified freezer_count() */
-	current->flags &= ~PF_FREEZER_SKIP;
+	freezer_count();
 }
 #endif