l3cfg: don't reset the ACD probe timestamp during timer events

acd_data->probing_timestamp_msec indicates when the probing
started. It is used in different places to calculate the timeout for
certain operations. In particular, it is used to detect that the probe
creation took too long when handling the ACD_STATE_CHANGE_MODE_TIMEOUT
event.

If we reset this timestamp at every timer event, we'll never hit the
probe creation timeout. Therefore, the l3cfg will keep trying forever
to create the probe.
See: https://lists.freedesktop.org/archives/networkmanager/2025-July/000418.html

Fix this by not updating the timestamp during a timeout event.

Fixes: a09f9cc616 ('l3cfg: ensure the probing timeout is initialized on probe start')
(cherry picked from commit 407d753a5a)

src/core/nm-l3cfg.c

@@ -2751,7 +2751,6 @@ handle_init:
                     goto handle_start_defending;
                 }
 
-                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;