mm: selftest to verify zero-filled pages are mapped to zeropage

When a THP is split, any subpage that is zero-filled will be mapped to the
shared zeropage, hence saving memory.  Add selftest to verify this by
allocating zero-filled THP and comparing RssAnon before and after split.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Alexander Zhu <[email protected]>
Signed-off-by: Usama Arif <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Cc: Barry Song <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Domenico Cerasuolo <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Kairui Song <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Nico Pache <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Shuang Zhai <[email protected]>
Cc: Yu Zhao <[email protected]>
Cc: Shuang Zhai <[email protected]>
Cc: Hugh Dickins <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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