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>

include/linux/kernel_stat.h

@@ -131,9 +131,9 @@ extern void account_steal_ticks(unsigned long ticks);
 extern void account_idle_ticks(unsigned long ticks);
 
 #ifdef CONFIG_VIRT_CPU_ACCOUNTING
-extern void account_switch_vtime(struct task_struct *prev);
+extern void vtime_task_switch(struct task_struct *prev);
 #else
-static inline void account_switch_vtime(struct task_struct *prev) { }
+static inline void vtime_task_switch(struct task_struct *prev) { }
 #endif
 
 #endif /* _LINUX_KERNEL_STAT_H */