mm/vmscan: add helper for querying ability to age anonymous pages

Anonymous pages are kept on their own LRU(s).  These lists could
theoretically always be scanned and maintained.  But, without swap, there
is currently nothing the kernel can *do* with the results of a scanned,
sorted LRU for anonymous pages.

A check for '!total_swap_pages' currently serves as a valid check as to
whether anonymous LRUs should be maintained.  However, another method will
be added shortly: page demotion.

Abstract out the 'total_swap_pages' checks into a helper, give it a
logically significant name, and check for the possibility of page
demotion.

[[email protected]: v11]
  Link: https://lkml.kernel.org/r/[email protected]
  Link: https://lkml.kernel.org/r/[email protected]

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Dave Hansen <[email protected]>
Signed-off-by: "Huang, Ying" <[email protected]>
Reviewed-by: Yang Shi <[email protected]>
Reviewed-by: Greg Thelen <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Wei Xu <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Keith Busch <[email protected]>
Cc: Yang Shi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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