Prevent race condition where gst_play_sink_do_reconfigure() could be called
from a pad probe while stream synchronizer pads are being released during
GST_STATE_CHANGE_PAUSED_TO_READY transition.
The race occurred when:
1. State change starts releasing stream synchronizer pads
2. Pads are unblocked earlier in the state change, allowing events to flow
3. A streaming thread triggers sinkpad_blocked_cb -> gst_play_sink_do_reconfigure
4. Reconfiguration tries to use already-released pad pointers
5. New pad creation fails with assertion in gst_pad_iterate_internal_links
The fix adds GST_PLAY_SINK_LOCK around the pad cleanup to ensure atomic
cleanup and prevent concurrent access during state transitions.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9233>
e799d79025