percpu_counters: use for_each_online_cpu()

Now that we have implemented hotunplug-time counter spilling,
percpu_counter_sum() only needs to look at online CPUs.

Cc: Gautham R Shenoy <ego@in.ibm.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>