mm, hotplug: move init_currently_empty_zone() under zone_span_lock protection

During online_pages phase, pgdat->nr_zones will be updated in case this
zone is empty.

Currently the online_pages phase is protected by the global locks
(device_device_hotplug_lock and mem_hotplug_lock), which ensures there is
no contention during the update of nr_zones.

These global locks introduces scalability issues (especially the second
one), which slow down code relying on get_online_mems().  This is also a
preparation for not having to rely on get_online_mems() but instead some
more fine grained locks.

The patch moves init_currently_empty_zone under both zone_span_writelock
and pgdat_resize_lock because both the pgdat state is changed (nr_zones)
and the zone's start_pfn.  Also this patch changes the documentation of
node_size_lock to include the protection of nr_zones.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Wei Yang <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Cc: David Hildenbrand <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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