mm: multigenerational lru: eviction

The eviction consumes old generations. Given an lruvec, the eviction
scans pages on lrugen->lists indexed by anon and file min_seq[2]
(modulo MAX_NR_GENS). It first tries to select a type based on the
values of min_seq[2]. If they are equal, it selects the type that has
a lower refault rate. The eviction sorts a page according to its
updated generation number if the aging has found this page accessed.
It also moves a page to the next generation if this page is from an
upper tier that has a higher refault rate than the base tier. The
eviction increments min_seq[2] of a selected type when it finds
lrugen->lists indexed by min_seq[2] of this selected type are empty.

With the aging and the eviction in place, implementing page reclaim
becomes quite straightforward:
  1) To reduce the latency, direct reclaim skips the aging unless both
  min_seq[2] are equal to max_seq-1. Then it invokes the eviction.
  2) To avoid the aging in the direct reclaim path, kswapd invokes the
  aging if either of min_seq[2] is equal to max_seq-1. Then it invokes
  the eviction.

Signed-off-by: Yu Zhao <yuzhao@google.com>
Tested-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>