cputime: Default implementation of nsecs -> cputime conversion

The architectures that override cputime_t (s390, ppc) don't provide
any version of nsecs_to_cputime(). Indeed this cputime_t implementation
by backend only happens when CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y under
which the core code doesn't make any use of nsecs_to_cputime().

At least for now.

We are going to make a broader use of it so lets provide a default
version with a per usecs granularity. It should be good enough for most
usecases.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
diff --git a/include/linux/kernel_stat.h b/include/linux/kernel_stat.h
index 51c72be..d7c6131 100644
--- a/include/linux/kernel_stat.h
+++ b/include/linux/kernel_stat.h
@@ -9,7 +9,7 @@
 #include <linux/sched.h>
 #include <linux/vtime.h>
 #include <asm/irq.h>
-#include <asm/cputime.h>
+#include <linux/cputime.h>
 
 /*
  * 'kernel_stat.h' contains the definitions needed for doing