Incrementally mark even if we have free pages

We move all pooled pages to free pages at the start of incremental
marking, so we shouldn't run incremental marking only when we have run
out of free pages. This causes incremental marking to always complete
in a single step.

gc.c

@@ -2347,7 +2347,7 @@ gc_continue(rb_objspace_t *objspace, rb_size_pool_t *size_pool, rb_heap_t *heap)
     gc_enter(objspace, gc_enter_event_continue, &lock_lev);
 
     /* Continue marking if in incremental marking. */
-    if (heap->free_pages == NULL && is_incremental_marking(objspace)) {
+    if (is_incremental_marking(objspace)) {
         if (gc_marks_continue(objspace, size_pool, heap)) {
             gc_sweep(objspace);
         }