mm/memory_hotplug: just build zonelist for newly added node

9adb62a5 ("mm/hotplug: correctly setup fallback zonelists when
creating new pgdat") tries to build the correct zonelist for a newly added
node, while it is not necessary to rebuild it for already exist nodes.

In build_zonelists(), it will iterate on nodes with memory.  For a newly
added node, it will have memory until node_states_set_node() is called in
online_pages().

This patch avoids rebuilding the zonelists for already existing nodes.

build_zonelists_node() uses managed_zone(zone) checks, so it should not
include empty zones anyway.  So effectively we avoid some pointless
work under stop_machine().

Link: http://lkml.kernel.org/r/20170626035822.50155-1-richard.weiyang@gmail.com


Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Jiang Liu <liuj97@gmail.com>
Cc: Xishi Qiu <qiuxishi@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>