edata_cache: Remember the associated base_t.

This will save us some trouble down the line when we stop passing arena pointers
everywhere; we won't have to pass around a base_t pointer either.

src/extent_dss.c

@@ -123,7 +123,7 @@ extent_alloc_dss(tsdn_t *tsdn, arena_t *arena, void *new_addr, size_t size,
 		return NULL;
 	}
 
-	gap = edata_cache_get(tsdn, &arena->edata_cache, arena->base);
+	gap = edata_cache_get(tsdn, &arena->edata_cache);
 	if (gap == NULL) {
 		return NULL;
 	}