cputime: Use a proper subsystem naming for vtime related APIs

Use a naming based on vtime as a prefix for virtual based
cputime accounting APIs:

- account_system_vtime() -> vtime_account()
- account_switch_vtime() -> vtime_task_switch()

It makes it easier to allow for further declension such
as vtime_account_system(), vtime_account_idle(), ... if we
want to find out the context we account to from generic code.

This also make it better to know on which subsystem these APIs
refer to.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index 305f23c..cab3da3d 100644
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -132,11 +132,11 @@
 struct task_struct;
 
 #if !defined(CONFIG_VIRT_CPU_ACCOUNTING) && !defined(CONFIG_IRQ_TIME_ACCOUNTING)
-static inline void account_system_vtime(struct task_struct *tsk)
+static inline void vtime_account(struct task_struct *tsk)
 {
 }
 #else
-extern void account_system_vtime(struct task_struct *tsk);
+extern void vtime_account(struct task_struct *tsk);
 #endif
 
 #if defined(CONFIG_TINY_RCU) || defined(CONFIG_TINY_PREEMPT_RCU)
@@ -162,7 +162,7 @@
  */
 #define __irq_enter()					\
 	do {						\
-		account_system_vtime(current);		\
+		vtime_account(current);		\
 		add_preempt_count(HARDIRQ_OFFSET);	\
 		trace_hardirq_enter();			\
 	} while (0)
@@ -178,7 +178,7 @@
 #define __irq_exit()					\
 	do {						\
 		trace_hardirq_exit();			\
-		account_system_vtime(current);		\
+		vtime_account(current);		\
 		sub_preempt_count(HARDIRQ_OFFSET);	\
 	} while (0)