mm/vmstat: add events for ksm cow

Users may use ksm by calling madvise(, , MADV_MERGEABLE) when they want to
save memory, it's a tradeoff by suffering delay on ksm cow.  Users can get
to know how much memory ksm saved by reading
/sys/kernel/mm/ksm/pages_sharing, but they don't know what's the costs of
ksm cow, and this is important of some delay sensitive tasks.

So add ksm cow events to help users evaluate whether or how to use ksm. 
Also update Documentation/admin-guide/mm/ksm.rst with new added events.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yang Yang <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: xu xin <[email protected]>
Reviewed-by: Ran Xiaokai <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Saravanan D <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: John Hubbard <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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