get_cpu_{idle,iowait}_time_us update idle/iowait counters unconditionally

get_cpu_{idle,iowait}_time_us update idle/iowait counters unconditionally
if the given CPU is in the idle loop.  This doesn't work well outside of
CPU governors which are singletons so nobody (except for IRQ) can race
with them.

We will need to use both functions from /proc/stat handler to properly
handle nohz idle/iowait times.

Let's update those counters only if the given last_update_time parameter
is non-NULL which means that the caller is interested in updating.

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>