When reactivating a cached page, reset the page's pool to the default

pool.  (Not doing this before was a performance pessimization but not
a cause for panic.)

sys/vm/vm_page.c

@@ -1044,6 +1044,7 @@ vm_page_alloc(vm_object_t object, vm_pindex_t pindex, int req)
 				return (NULL);
 			}
 			vm_phys_unfree_page(m);
+			vm_phys_set_pool(VM_FREEPOOL_DEFAULT, m, 0);
 		} else if ((req & VM_ALLOC_IFCACHED) != 0) {
 			mtx_unlock(&vm_page_queue_free_mtx);
 			return (NULL);