Fix a bug in arena_dalloc_bin_run().

Fix the newsize argument to arena_run_trim_tail() that
arena_dalloc_bin_run() passes.  Previously, oldsize-newsize (i.e. the
complement) was passed, which could erroneously cause dirty pages to be
returned to the clean available runs tree.  Prior to the
CHUNK_MAP_ZEROED --> CHUNK_MAP_UNZEROED conversion, this bug merely
caused dirty pages to be unaccounted for (and therefore never get
purged), but with CHUNK_MAP_UNZEROED, this could cause dirty pages to be
treated as zeroed (i.e. memory corruption).