vtime: Always debug check snapshot source _before_ updating it

The vtime delta update performed by get_vtime_delta() always check
that the source of the snapshot is valid.

Meanhile the snapshot updaters that rely on get_vtime_delta() also
set the new snapshot origin. But some of them do this right before
the call to get_vtime_delta(), making its debug check useless.

This is easily fixable by moving the snapshot origin update after
the call to get_vtime_delta(). The order doesn't matter there.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Kevin Hilman <khilman@linaro.org>

kernel/sched/cputime.c

@@ -660,9 +660,9 @@ void vtime_account_system(struct task_struct *tsk)
 void vtime_gen_account_irq_exit(struct task_struct *tsk)
 {
 	write_seqlock(&tsk->vtime_seqlock);
+	__vtime_account_system(tsk);
 	if (context_tracking_in_user())
 		tsk->vtime_snap_whence = VTIME_USER;
-	__vtime_account_system(tsk);
 	write_sequnlock(&tsk->vtime_seqlock);
 }
 
@@ -680,8 +680,8 @@ void vtime_account_user(struct task_struct *tsk)
 void vtime_user_enter(struct task_struct *tsk)
 {
 	write_seqlock(&tsk->vtime_seqlock);
-	tsk->vtime_snap_whence = VTIME_USER;
 	__vtime_account_system(tsk);
+	tsk->vtime_snap_whence = VTIME_USER;
 	write_sequnlock(&tsk->vtime_seqlock);
 }