mm/idle_page_tracking: make PG_idle reusable

PG_idle and PG_young allow the two PTE Accessed bit users, Idle Page
Tracking and the reclaim logic concurrently work while not interfering
with each other.  That is, when they need to clear the Accessed bit, they
set PG_young to represent the previous state of the bit, respectively.
And when they need to read the bit, if the bit is cleared, they further
read the PG_young to know whether the other has cleared the bit meanwhile
or not.

For yet another user of the PTE Accessed bit, we could add another page
flag, or extend the mechanism to use the flags.  For the DAMON usecase,
however, we don't need to do that just yet.  IDLE_PAGE_TRACKING and DAMON
are mutually exclusive, so there's only ever going to be one user of the
current set of flags.

In this commit, we split out the CONFIG options to allow for the use of
PG_young and PG_idle outside of idle page tracking.

In the next commit, DAMON's reference implementation of the virtual memory
address space monitoring primitives will use it.

[[email protected]: set PAGE_EXTENSION for non-64BIT]
  Link: https://lkml.kernel.org/r/[email protected]
[[email protected]: tweak Kconfig text]
[[email protected]: hide PAGE_IDLE_FLAG from users]
  Link: https://lkml.kernel.org/r/[email protected]

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: SeongJae Park <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Reviewed-by: Fernand Sieber <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Amit Shah <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Brendan Higgins <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Fan Du <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Greg Thelen <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Jonathan Cameron <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Leonard Foerster <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: Markus Boehme <[email protected]>
Cc: Maximilian Heyne <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Steven Rostedt (VMware) <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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