selftest: add testing unsharing and counting ksm zero page

selftest: add testing unsharing and counting ksm zero page

Add a function test_unmerge_zero_page() to test the functionality on
unsharing and counting ksm-placed zero pages and counting of this patch
series.

test_unmerge_zero_page() actually contains three subjct test objects:
	1) whether the count of ksm zero page can react correctly to cow
	   (copy on write);
	2) whether the count of ksm zero page can react correctly to unmerge;
	3) whether ksm zero pages are really unmerged.

Link: https://lkml.kernel.org/r/202212300918477352037@zte.com.cn
Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Reviewed-by: Xiaokai Ran <ran.xiaokai@zte.com.cn>
Reviewed-by: Yang Yang <yang.yang29@zte.com.cn>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Xuexin Jiang <jiang.xuexin@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>