mm: multigenerational lru: aging

The aging produces young generations. Given an lruvec, the aging walks
the mm_struct list associated with this lruvec to scan page tables for
referenced pages. Upon finding one, the aging updates the generation
number of this page to max_seq. After each round of scan, the aging
increments max_seq. The aging is due when both of min_seq[2] reaches
max_seq-1, assuming both anon and file types are reclaimable.

The aging uses the following optimizations when scanning page tables:
  1) It will not scan page tables from processes that have been
  sleeping since the last scan.
  2) It will not scan PTE tables under non-leaf PMD entries that do
  not have the accessed bit set, when
  CONFIG_HAVE_ARCH_PARENT_PMD_YOUNG=y.
  3) It will not zigzag between the PGD table and the same PMD or PTE
  table spanning multiple VMAs. In other words, it finishes all the
  VMAs with the range of the same PMD or PTE table before it returns
  to the PGD table. This optimizes workloads that have large numbers
  of tiny VMAs, especially when CONFIG_PGTABLE_LEVELS=5.

Signed-off-by: Yu Zhao <yuzhao@google.com>