selftest-add-testing-unsharing-and-counting-ksm-zero-page-v6

selftest-add-testing-unsharing-and-counting-ksm-zero-page-v6

v5->v6:
According to David's suggestions, the following changes are made:
1) Rename check_ksm_zero_pages_count() -> ksm_get_zero_pages(), and do the
   comparison outside.
2) Open all global fd from main() rather than the test case.
3) Remove COW-related test codes and focus on explicit unmerging here.
4) Add some coments to explain why wait_two_full_scans is required.
5) Clean up some unneed changes.

Link: https://lkml.kernel.org/r/202302100921574141612@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>