mm: set N_CPU to node_states during boot

After a system booted, N_CPU is not set to any node as has_cpu shows an
empty line.

  # cat /sys/devices/system/node/has_cpu
  (show-empty-line)

setup_vmstat() registers its CPU notifier callback,
vmstat_cpuup_callback(), which marks N_CPU to a node when a CPU is put
into online.  However, setup_vmstat() is called after all CPUs are
launched in the boot sequence.

Changed setup_vmstat() to mark N_CPU to the nodes with online CPUs at
boot, which is consistent with other operations in
vmstat_cpuup_callback(), i.e.  start_cpu_timer() and
refresh_zone_stat_thresholds().

Also added get_online_cpus() to protect the for_each_online_cpu() loop.

Signed-off-by: Toshi Kani <toshi.kani@hp.com>
Acked-by: Christoph Lameter <cl@linux.com>
Reviewed-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Tested-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>