l3cfg: ensure the probing timeout is initialized on probe start

When handling event TIMEOUT, "acd_data->probing_timeout_msec" needs to
be always initialized before jumping to "handle_start_probing:";
otherwise, an assertion failure is triggered at:

  static void
  _l3_acd_data_timeout_schedule_probing_restart(AcdData *acd_data, gint64 now_msec)
  {
    ...
    nm_assert(acd_data->probing_timeout_msec > 0);

Even if the ACD data is already in state PROBE, that doesn't mean that
the timeout is already initialized because the PROBE state can also be
reached from a INSTANCE_RESET event; and depending on the previous
state "acd_data->probing_timeout_msec" could be uninitialized.

Fixes-test: @iptunnel_restart
Fixes: b8f9d7b5dd ('l3cfg: rework ACD handling in NML3Cfg to support handling conflicts')

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/2023

src/core/nm-l3cfg.c

@@ -2752,7 +2752,9 @@ handle_init:
                     goto handle_start_defending;
                 }
 
-                log_reason = "retry probing on timeout";
+                acd_data->probing_timestamp_msec = (*p_now_msec);
+                acd_data->probing_timeout_msec   = acd_timeout_msec;
+                log_reason                       = "retry probing on timeout";
                 goto handle_start_probing;
             }