Remove internal __mode = "K" and replace with safe check.

src/lib_ffi.c

@@ -811,7 +811,7 @@ static GCtab *ffi_finalizer(lua_State *L)
   settabV(L, L->top++, t);
   setgcref(t->metatable, obj2gco(t));
   setstrV(L, lj_tab_setstr(L, t, lj_str_newlit(L, "__mode")),
-	  lj_str_newlit(L, "K"));
+	  lj_str_newlit(L, "k"));
   t->nomm = (uint8_t)(~(1u<<MM_mode));
   return t;
 }