mm, vmscan: avoid passing in classzone_idx unnecessarily to shrink_node

mm, vmscan: avoid passing in classzone_idx unnecessarily to shrink_node

shrink_node receives all information it needs about classzone_idx from
sc->reclaim_idx so remove the aliases.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Hillf Danton <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Rik van Riel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>