Revert "copperplate/heapobj-pshared: fix memory limit"

This reverts commit 5b0c3677 which was
wrong and fortunately did not survive the upcoming memcheck tests.

lib/copperplate/heapobj-pshared.c

@@ -226,7 +226,7 @@ static int init_heap(struct shared_heap *heap, void *base,
 	extent->bitmap = __shoff(base, mem) + bmapoff;
 	extent->bitwords = bmapwords;
 	extent->membase = __shoff(base, mem) + metasz;
-	extent->memlim = __shoff(base, mem) + size;
+	extent->memlim = extent->membase + size;
 	init_extent(base, extent);
 	__list_append(base, &extent->link, &heap->extents);