Fix performance regression in arena_palloc().

Pass large allocation requests to arena_malloc() when possible.  This
regression was introduced by 155bfa7da18cab0d21d87aa2dce4554166836f5d
(Normalize size classes.).

Bug: 21326736
(cherry picked from commit 2bb0b03fd3c2e72d21505db1f813362c6204d18f)

Change-Id: I305acf080a91f1448e3fc4a602712d4f240ef367
1 file changed