ksm: add ksm zero pages for each process

As the number of ksm zero pages is not included in ksm_merging_pages per
process when enabling use_zero_pages, it's unclear of how many actual
pages are merged by KSM. To let users accurately estimate their memory
demands when unsharing KSM zero-pages, it's necessary to show KSM zero-
pages per process. In addition, it help users to know the actual KSM
profit because KSM-placed zero pages are also benefit from KSM.

since unsharing zero pages placed by KSM accurately is achieved, then
tracking empty pages merging and unmerging is not a difficult thing any
longer.

Since we already have /proc/<pid>/ksm_stat, just add the information of
'ksm_zero_pages' in it.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: xu xin <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Reviewed-by: Xiaokai Ran <[email protected]>
Reviewed-by: Yang Yang <[email protected]>
Cc: Claudio Imbrenda <[email protected]>
Cc: Xuexin Jiang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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