mdns: fix possible deadlock on mdns deinit calling mdns_free()

mdns: fix possible deadlock on mdns deinit calling mdns_free()

mnds_free() initiates stop and delete timer tasks, which after locking the mutex could lead to a dead lock in case timer task executed before deleting the task, as it would wait indefinitelly for unlocking the mutex. This condition is fixed by calling _mdns_stop_timer without locking the mutex, because there's no need to protect any data when stopping and deleting the timer task

Closes https://github.com/espressif/esp-idf/issues/1696