mm/vmstat: Convert to hotplug state machine

Install the callbacks via the state machine, but do not invoke them as we
can initialize the node state without calling the callbacks on all online
CPUs.

start_shepherd_timer() is now called outside the get_online_cpus() block
which is safe as it only operates on cpu possible mask.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Cc: rt@linutronix.de
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Link: http://lkml.kernel.org/r/20161129145221.ffc3kg3hd7lxiwj6@linutronix.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>