show_stat handler of the /proc/stat file relies on kstat_cpu(cpu)

show_stat handler of the /proc/stat file relies on kstat_cpu(cpu)
statistics when priting information about idle and iowait times.  This is
OK if we are not using tickless kernel (CONFIG_NO_HZ) because counters are
updated periodically.

With NO_HZ things got more tricky because we are not doing idle/iowait
accounting while we are tickless so the value might get outdated.  Users
of /proc/stat will notice that by unchanged idle/iowait values which is
then interpreted as 0% idle/iowait time.  From the user space POV this is
an unexpected behavior and a change of the interface.

Let's fix this by using get_cpu_{idle,iowait}_time_us which accounts the
total idle/iowait time since boot and it doesn't rely on sampling or any
other periodic activity.  Fall back to the previous behavior if NO_HZ is
disabled or not configured.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Dave Jones <davej@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>