mem-hotplug: improve zone_movable_is_highmem logic

In original code, zone_movable_is_highmem() assumes ZONE_MOVABLE not
highmem if CONFIG_HAVE_MEMBLOCK_NODE_MAP is not set.  In online_pages,
it extracts pages from the previous zone before ZONE_MOVABLE.  Which is
logically inconsistent:

If HAVE_MEMBLOCK_NODE_MAP is turned off but HIGHMEM is on,
zone_movable_is_highmem() makes movable zone not highmem, but
online_pages() extracts pages from ZONE_HIGHMEM.

This inconsistency doesn't cause real problem currently, because all
architectures support online_pages also have HAVE_MEMBLOCK_NODE_MAP.
However, fixing it makes code clear, and also helps futher coding.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Zhang Zhen <zhangzhen@huawei.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Jiang Liu <liuj97@gmail.com>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>