Replace buffer lock magic numbers with defines (#5217)

* replace magic numbers with defines

wled00/xml.cpp

@@ -99,7 +99,7 @@ void appendGPIOinfo(Print& settingsScript)
     settingsScript.printf_P(PSTR(",%d,%d"), spi_mosi, spi_sclk);
   }
   // usermod pin reservations will become unnecessary when settings pages will read cfg.json directly
-  if (requestJSONBufferLock(6)) {
+  if (requestJSONBufferLock(JSON_LOCK_XML)) {
     // if we can't allocate JSON buffer ignore usermod pins
     JsonObject mods = pDoc->createNestedObject("um");
     UsermodManager::addToConfig(mods);