Bug fix: Do not split pages when freeing pagetable pages.

Bug fix: Do not split pages when freeing pagetable pages.

The old way we free pagetable pages was wrong.

The old way is:
When we got a hugepage, we split it into smaller pages. And sometimes,
we only need to free some of the smaller pages because the others are
still in use. And the whole larger page will be freed if all the smaller
pages are not in use. All these were done in remove_pmd/pud_table().

But there is no way to know if the larger page has been split. As a result,
there is no way to decide when to split.

Actually, there is no need to split larger pages into smaller ones.

We do it in the following new way:
1) For direct mapped pages, all the pages were freed when they were offlined.
   And since menmory offline is done section by section, all the memory ranges
   being removed are aligned to PAGE_SIZE. So only need to deal with unaligned
   pages when freeing vmemmap pages.

2) For vmemmap pages being used to store page_struct, if part of the larger
   page is still in use, just fill the unused part with 0xFD. And when the
   whole page is fulfilled with 0xFD, then free the larger page.

This problem is caused by the following related patch:
memory-hotplug-common-apis-to-support-page-tables-hot-remove.patch
memory-hotplug-common-apis-to-support-page-tables-hot-remove-fix.patch
memory-hotplug-common-apis-to-support-page-tables-hot-remove-fix-fix.patch
memory-hotplug-common-apis-to-support-page-tables-hot-remove-fix-fix-fix.patch
memory-hotplug-common-apis-to-support-page-tables-hot-remove-fix-fix-fix-fix.patch

This patch will fix the problem based on the above patches.

Reported-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Cc: Jiang Liu <jiang.liu@huawei.com>
Cc: Jianguo Wu <wujianguo@huawei.com>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>