mm/ksm: optimize the chain()/chain_prune() interfaces

Now the implementation of stable_node_dup() causes chain()/chain_prune()
interfaces and usages are overcomplicated.

Why?  stable_node_dup() only find and return a candidate stable_node for
sharing, so the users have to recheck using stable_node_dup_any() if any
non-candidate stable_node exist.  And try to ksm_get_folio() from it
again.

Actually, stable_node_dup() can just return a best stable_node as it can,
then the users can check if it's a candidate for sharing or not.

The code is simplified too and fewer corner cases: such as stable_node and
stable_node_dup can't be NULL if returned tree_folio is not NULL.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Chengming Zhou <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Stefan Roesch <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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