uma: fix order to finit/dtor in uma_zfree()

the mbuf ext buffer is freed in the dtor, so it should be called before finit.
this is fixing a crash that surfaced by using the conf-memory-debug=1

bsd/porting/uma_stub.c

@@ -46,14 +46,14 @@ void uma_zfree_arg(uma_zone_t zone, void *item, void *udata)
         return;
     }
 
-    if (zone->uz_fini) {
-        zone->uz_fini(item, zone->uz_size);
-    }
-
     if (zone->uz_dtor) {
         zone->uz_dtor(item, zone->uz_size, udata);
     }
 
+    if (zone->uz_fini) {
+        zone->uz_fini(item, zone->uz_size);
+    }
+
     free(item);
 }