[RFC PATCH 5/10] use an indexed array for LRU lists and variables

[Rik van Riel]

Use an indexed array for LRU variables.  This makes the rest
of the split VM code a lot cleaner.

V1 -> V2 [lts]:
+ Remove extraneous  __dec_zone_state(zone, NR_ACTIVE) pointed
  out by Mel G.